Molecules that bind to follistatin polypeptides

Abstract

This document provides methods and materials involved in binding a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) to a follistatin polypeptide. For example, binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, cell engagers, and / or ADCs) that bind to a follistatin polypeptide and methods and materials for using one or more such binding molecules to treat a mammal (e.g., a human) having cancer are provided.

Description

[0001]Attorney Docket No.48881-0047WO1 / 05958 MOLECULES THAT BIND TO FOLLISTATIN POLYPEPTIDES CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from U.S. Provisional Application Serial No. 63 / 442,943, filed February 2, 2023. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application. SEQUENCE LISTING This application contains a Sequence Listing that has been submitted electronically as an XML file named “48881-0048WO1_SL.xml.” The XML file, created on January 29, 2024, is 581,555 bytes in size. The material in the XML file is hereby incorporated by reference in its entirety. BACKGROUND 1. Technical Field This document relates to methods and materials involved in binding a molecule (e.g., an antibody, a fragment of an antibody, an antibody domain, a chimeric antigen receptor (CAR), a cell engager, or an antibody-drug conjugate (ADC)) to a follistatin polypeptide. For example, this document provides binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, cell engagers, or ADCs) that bind to a follistatin polypeptide and methods and materials for using such binders alone or in combination with one or more chemotherapeutic agents to treat cancer. This document also provides cells (e.g., host cells) designed to express one or more binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, or cell engagers) having the ability to bind to a follistatin polypeptide and methods and materials for using such cells to treat cancer. 2. Background Information Follistatin (FST), also known as activin-binding protein, is an extracellular glycoprotein (with a size of about 35 kD of the most dominant form) encoded in humans Attorney Docket No.48881-0047WO1 / 05958 by the FST gene. FST was identified as an inhibitor of pituitary follicle stimulating hormone secretion. FST primarily functions in binding and neutralization of members of the TGF-? superfamily, with a particular focus on activin, which is involved in proliferation, differentiation, and apoptosis of a number of cell types. FST is involved in tumorigenesis, metastasis, and angiogenesis of solid tumors through its interaction with activin. High serum levels of follistatin polypeptides were reported in patients with ovarian cancer, indicating that follistatin polypeptides can be a potential tumor marker for ovarian cancer diagnosis. Thus, follistatin polypeptides are a potential target for treating different types of cancers. SUMMARY This document provides methods and materials involved in binding a molecule (e.g., an antibody, an antigen binding fragment, an antibody domain, a CAR, a cell engager, or an ADC) to a follistatin polypeptide. For example, this document provides binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, cell engagers, or ADCs) that bind to a follistatin polypeptide and methods and materials for using one or more such binders to treat a mammal (e.g., a human) having cancer (e.g., ovarian cancer, lung cancer, and / or prostate cancer). In some cases, this document provides methods and materials for using one or more binders described herein in combination with one or more chemotherapeutic agents to treat a mammal (e.g., a human) having cancer (e.g., ovarian cancer, lung cancer, and / or prostate cancer). This document also provides cells (e.g., host cells) designed to express one or more binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, or cell engagers) having the ability to bind to a follistatin polypeptide and methods and materials for using such cells to treat cancer (e.g., ovarian cancer, lung cancer, and / or prostate cancer). In some cases, this document provides methods and materials for using one or more of such cells in combination with one or more chemotherapeutic agents to treat a mammal (e.g., a human) having cancer (e.g., ovarian cancer, lung cancer, and / or prostate cancer). Attorney Docket No.48881-0047WO1 / 05958 As described herein, binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more CARs, one or more cell engagers, and / or one or more ADCs) can be designed to have the ability to bind to a follistatin polypeptide. For example, a binder (e.g., an antibody, an antigen binding fragment, an antibody domain, a CAR, a cell engager, or an ADC) provided herein can have the ability to bind to a polypeptide comprising, consisting essentially of, or consisting of the amino acid sequence of a human follistatin polypeptide as set forth in SEQ ID NO:113 and / or SEQ ID NO:115 (see, e.g., Example 2). As described herein, anti-follistatin binders (e.g., Clones #1-#14) bind to follistatin polypeptides (e.g., human follistatin polypeptides). See, e.g., Figures 4-6. In addition, exposing cancer cells to an anti-follistatin binder described herein (e.g., any one of Clones #1-#14) and a chemotherapeutic agent (e.g., taxol or cisplatin) enhance the ability of the chemotherapeutic agent to kill cancer cells. See, e.g., Figures 7-9. These results demonstrate that anti-follistatin binders provided herein can be used to treat cancer (e.g., ovarian cancer, lung cancer, and / or prostate cancer). In some cases, a single set of three CDRs of an antibody domain (e.g., a VH domain) provided herein (e.g., SEQ ID NOs:1-3; SEQ ID NOs:9-11; SEQ ID NOs:17-19; SEQ ID NOs:25-27; SEQ ID NOs:33-35; SEQ ID NOs:41-43; SEQ ID NOs:49-51; SEQ ID NOs:57-59; SEQ ID NOs:65-67; SEQ ID NOs:73-75; SEQ ID NOs:81-83; SEQ ID NOs:89-91; SEQ ID NOs:97-99; or SEQ ID NOs:105-107) can be engineered into a CAR to create CAR+cells (e.g., CAR+T cells, CAR+stem cells such as CAR+induced pluripotent stem cells, or CAR+NK cells) having the ability to target follistatin+cells (e.g., follistatin+tumor cells and / or follistatin+tumor vasculature), can be engineered into an antibody structure that includes an Fc region to create antibodies having the ability to target follistatin+cells (e.g., follistatin+tumor cells and / or follistatin+tumor vasculature) and induce ADCC against the target follistatin+cells, and / or can be engineered into a cell engager such as a bi-specific T cell engager (e.g., a BiTE), a bi-specific killer engager (e.g., a BiKE), and / or a tri-specific killer engager (e.g., a TriKE) to create cell engagers having the ability to target follistatin+cells (e.g., follistatin+tumor cells and / or follistatin+tumor vasculature) and induce one or more immune responses (e.g., T cell immune Attorney Docket No.48881-0047WO1 / 05958 responses and / or ADCC using a cell engager in the absence of an Fc-containing antibody) against the target follistatin+cells. It is noted that BiKE- and TriKE-mediated killing can be referred to as ADCC even though it is not initiated by an Fc domain in cases when the engager lacks an Fc domain. In addition, as described herein, binders (e.g., one or more antibodies, one or more antigen binding fragments, and / or one or more antibody domains) provided herein can be used to create conjugates that include the binder and a drug. For example, ADCs such as full antibody-drug conjugates, Fab-drug conjugates, scFv-drug conjugates, and / or antibody domain-drug conjugates can be designed to include an appropriate binder provided herein to create the conjugate. Such conjugates can be used to deliver the drug payload to target cells such as cancer cells (e.g., follistatin+cancer cells) or cancer vasculature (e.g., follistatin+cancer vasculature). As also described herein, binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein can be used to treat a mammal (e.g., a human) having cancer. For example, a mammal (e.g., a human) having cancer (e.g., a follistatin+cancer) can be administered a composition comprising one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) described herein to reduce the number of cancer cells within the mammal, to induce ADCC against cancer cells within the mammal, and / or to increase the survival duration of the mammal from cancer (e.g., ovarian cancer, lung cancer, and / or prostate cancer). As also described herein, cells (e.g., host cells) can be designed to express one or more binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, or cell engagers) having the ability to bind to a follistatin polypeptide. For example, cells such as T cells (e.g., CTLs), stem cells (e.g., induced pluripotent stem cells), or NK cells can be engineered to express one or more CARs having the ability to bind to a follistatin polypeptide. Such cells (e.g., follistatin-specific CAR+T cells or NK cells) can be used to treat cancer (e.g., ovarian cancer, lung cancer, and / or prostate cancer). Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein can be used to detect the presence or absence of a follistatin polypeptide. For example, a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein can be used to determine whether or not a sample (e.g., a biological sample such tumor biopsy) obtained from a mammal (e.g., a human) contains follistatin+cells (e.g., follistatin+cancer cells). Having the ability to detect the presence or absence of a follistatin polypeptide (e.g., follistatin+cancer cells) can allow clinicians, health professionals, and patients to make better decisions about possible treatment options. For example, detection of follistatin+cancer cells within a mammal can allow clinicians, health professionals, and patients to select an appropriate anti-cancer treatment that targets the follistatin+cancer cells. Such treatments that target the follistatin+cancer cells can include administration of one or more of the binders described herein having the ability to bind to a follistatin polypeptide and / or administration of one or more cells (e.g., follistatin-specific CAR+T cells or NK cells) designed to express a binder described herein. In general, one aspect of this document features an antibody comprising (or consisting essentially of, or consisting of) (i) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:1 (or SEQ ID NO:1 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:2 (or SEQ ID NO:2 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:3 (or SEQ ID NO:3 with one, two, or three amino acid additions, deletions, or substitutions); (ii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:9 (or SEQ ID NO:9 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:10 (or SEQ ID NO:10 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:11 (or SEQ ID NO:11 with one, two, or three amino acid additions, deletions, or substitutions); (iii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:17 (or SEQ ID NO:17 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:18 (or SEQ ID NO:18 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:19 (or Attorney Docket No.48881-0047WO1 / 05958 SEQ ID NO:19 with one, two, or three amino acid additions, deletions, or substitutions); (iv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:25 (or SEQ ID NO:25 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:26 (or SEQ ID NO:26 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:27 (or SEQ ID NO:27 with one, two, or three amino acid additions, deletions, or substitutions); (v) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:33 (or SEQ ID NO:33 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:34 (or SEQ ID NO:34 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:35 (or SEQ ID NO:35 with one, two, or three amino acid additions, deletions, or substitutions); (vi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:41 (or SEQ ID NO:41 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:42 (or SEQ ID NO:42 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:43 (or SEQ ID NO:43 with one, two, or three amino acid additions, deletions, or substitutions); (vii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:49 (or SEQ ID NO:49 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:50 (or SEQ ID NO:50 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:51 (or SEQ ID NO:51 with one, two, or three amino acid additions, deletions, or substitutions); (viii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:57 (or SEQ ID NO:57 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:58 (or SEQ ID NO:58 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:59 (or SEQ ID NO:59 with one, two, or three amino acid additions, deletions, or substitutions); (ix) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:65 (or SEQ ID NO:65 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:66 (or SEQ ID NO:66 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:67 (or SEQ ID NO:67 with one, two, or Attorney Docket No.48881-0047WO1 / 05958 three amino acid additions, deletions, or substitutions); (x) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:73 (or SEQ ID NO:73 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:74 (or SEQ ID NO:74 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:75 (or SEQ ID NO:75 with one, two, or three amino acid additions, deletions, or substitutions); (xi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:81 (or SEQ ID NO:81 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:82 (or SEQ ID NO:82 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:83 (or SEQ ID NO:83 with one, two, or three amino acid additions, deletions, or substitutions); (xii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:89 (or SEQ ID NO:89 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:90 (or SEQ ID NO:90 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:91 (or SEQ ID NO:91 with one, two, or three amino acid additions, deletions, or substitutions); (xiii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:97 (or SEQ ID NO:97 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:98 (or SEQ ID NO:98 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:99 (or SEQ ID NO:99 with one, two, or three amino acid additions, deletions, or substitutions); or (xiv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:105 (or SEQ ID NO:105 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:106 (or SEQ ID NO:106 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:107 (or SEQ ID NO:107 with one, two, or three amino acid additions, deletions, or substitutions). The antibody can comprise the ability to bind to SEQ ID NO:113 and / or SEQ ID NO:115. The antibody can comprise the heavy chain variable domain or region of (i). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:8. The antibody can comprise the heavy chain variable domain or region of (ii). The heavy Attorney Docket No.48881-0047WO1 / 05958 chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:16. The antibody can comprise the heavy chain variable domain or region of (iii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:24. The antibody can comprise the heavy chain variable domain or region of (iv). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:32. The antibody can comprise the heavy chain variable domain or region of (v). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:40. The antibody can comprise the heavy chain variable domain or region of (vi). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:48. The antibody can comprise the heavy chain variable domain or region of (vii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:56. The antibody can comprise the heavy chain variable domain or region of (viii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:64. The antibody can comprise the heavy chain variable domain or region of (ix). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:72. The antibody can comprise the heavy chain variable domain or region of (x). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:80. The antibody can comprise the heavy chain variable domain or region of (xi). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:88. The antibody can comprise the heavy chain variable domain or region of (xii). The heavy chain variable domain or region can Attorney Docket No.48881-0047WO1 / 05958 comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:96. The antibody can comprise the heavy chain variable domain or region of (xiii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:104. The antibody can comprise the heavy chain variable domain or region of (xiv). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:112. The antibody can be a monoclonal antibody. The antibody can be an scFv antibody. This paragraph can be referred to as first aspect paragraph. In another aspect, this document features an antigen binding fragment comprising (or consisting essentially of, or consisting of): (i) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:1 (or SEQ ID NO:1 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:2 (or SEQ ID NO:2 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:3 (or SEQ ID NO:3 with one, two, or three amino acid additions, deletions, or substitutions); (ii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:9 (or SEQ ID NO:9 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:10 (or SEQ ID NO:10 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:11 (or SEQ ID NO:11 with one, two, or three amino acid additions, deletions, or substitutions); (iii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:17 (or SEQ ID NO:17 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:18 (or SEQ ID NO:18 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:19 (or SEQ ID NO:19 with one, two, or three amino acid additions, deletions, or substitutions); (iv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:25 (or SEQ ID NO:25 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:26 (or SEQ ID NO:26 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:27 (or SEQ ID NO:27 Attorney Docket No.48881-0047WO1 / 05958 with one, two, or three amino acid additions, deletions, or substitutions); (v) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:33 (or SEQ ID NO:33 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:34 (or SEQ ID NO:34 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:35 (or SEQ ID NO:35 with one, two, or three amino acid additions, deletions, or substitutions); (vi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:41 (or SEQ ID NO:41 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:42 (or SEQ ID NO:42 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:43 (or SEQ ID NO:43 with one, two, or three amino acid additions, deletions, or substitutions); (vii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:49 (or SEQ ID NO:49 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:50 (or SEQ ID NO:50 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:51 (or SEQ ID NO:51 with one, two, or three amino acid additions, deletions, or substitutions); (viii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:57 (or SEQ ID NO:57 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:58 (or SEQ ID NO:58 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:59 (or SEQ ID NO:59 with one, two, or three amino acid additions, deletions, or substitutions); (ix) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:65 (or SEQ ID NO:65 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:66 (or SEQ ID NO:66 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:67 (or SEQ ID NO:67 with one, two, or three amino acid additions, deletions, or substitutions); (x) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:73 (or SEQ ID NO:73 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:74 (or SEQ ID NO:74 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:75 (or SEQ ID NO:75 with one, two, or three amino acid Attorney Docket No.48881-0047WO1 / 05958 additions, deletions, or substitutions); (xi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:81 (or SEQ ID NO:81 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:82 (or SEQ ID NO:82 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:83 (or SEQ ID NO:83 with one, two, or three amino acid additions, deletions, or substitutions); (xii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:89 (or SEQ ID NO:89 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:90 (or SEQ ID NO:90 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:91 (or SEQ ID NO:91 with one, two, or three amino acid additions, deletions, or substitutions); (xiii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:97 (or SEQ ID NO:97 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:98 (or SEQ ID NO:98 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:99 (or SEQ ID NO:99 with one, two, or three amino acid additions, deletions, or substitutions); or (xiv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:105 (or SEQ ID NO:105 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:106 (or SEQ ID NO:106 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:107 (or SEQ ID NO:107 with one, two, or three amino acid additions, deletions, or substitutions). The antigen binding fragment can comprise the ability to bind to SEQ ID NO:113 and / or SEQ ID NO:115. The antigen binding fragment can comprise the heavy chain variable domain or region of (i). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:8. The antigen binding fragment can comprise the heavy chain variable domain or region of (ii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:16. The antigen binding fragment can comprise the heavy chain variable domain or region of (iii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the Attorney Docket No.48881-0047WO1 / 05958 amino acid sequence set forth in SEQ ID NO:24. The antigen binding fragment can comprise the heavy chain variable domain or region of (iv). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:32. The antigen binding fragment can comprise the heavy chain variable domain or region of (v). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:40. The antigen binding fragment can comprise the heavy chain variable domain or region of (vi). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:48. The antigen binding fragment can comprise the heavy chain variable domain or region of (vii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:56. The antigen binding fragment can comprise the heavy chain variable domain or region of (viii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:64. The antigen binding fragment can comprise the heavy chain variable domain or region of (ix). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:72. The antigen binding fragment can comprise the heavy chain variable domain or region of (x). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:80. The antigen binding fragment can comprise the heavy chain variable domain or region of (xi). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:88. The antigen binding fragment can comprise the heavy chain variable domain or region of (xii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:96. The antigen binding fragment can comprise the heavy chain variable domain or region of (xiii). The heavy chain variable domain or region can comprise an amino acid sequence Attorney Docket No.48881-0047WO1 / 05958 having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:104. The antigen binding fragment can comprise the heavy chain variable domain or region of (xiv). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:112. The antigen binding fragment can be monoclonal. The antigen binding fragment can be a Fab. This paragraph can be referred to as second aspect paragraph. In another aspect, this document features an antibody domain comprising (or consisting essentially of, or consisting of): (i) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:1 (or SEQ ID NO:1 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:2 (or SEQ ID NO:2 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:3 (or SEQ ID NO:3 with one, two, or three amino acid additions, deletions, or substitutions); (ii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:9 (or SEQ ID NO:9 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:10 (or SEQ ID NO:10 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:11 (or SEQ ID NO:11 with one, two, or three amino acid additions, deletions, or substitutions); (iii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:17 (or SEQ ID NO:17 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:18 (or SEQ ID NO:18 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:19 (or SEQ ID NO:19 with one, two, or three amino acid additions, deletions, or substitutions); (iv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:25 (or SEQ ID NO:25 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:26 (or SEQ ID NO:26 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:27 (or SEQ ID NO:27 with one, two, or three amino acid additions, deletions, or substitutions); (v) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:33 (or SEQ ID NO:33 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:34 (or SEQ ID NO:34 with one, two, or three amino acid Attorney Docket No.48881-0047WO1 / 05958 additions, deletions, or substitutions), and SEQ ID NO:35 (or SEQ ID NO:35 with one, two, or three amino acid additions, deletions, or substitutions); (vi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:41 (or SEQ ID NO:41 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:42 (or SEQ ID NO:42 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:43 (or SEQ ID NO:43 with one, two, or three amino acid additions, deletions, or substitutions); (vii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:49 (or SEQ ID NO:49 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:50 (or SEQ ID NO:50 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:51 (or SEQ ID NO:51 with one, two, or three amino acid additions, deletions, or substitutions); (viii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:57 (or SEQ ID NO:57 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:58 (or SEQ ID NO:58 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:59 (or SEQ ID NO:59 with one, two, or three amino acid additions, deletions, or substitutions); (ix) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:65 (or SEQ ID NO:65 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:66 (or SEQ ID NO:66 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:67 (or SEQ ID NO:67 with one, two, or three amino acid additions, deletions, or substitutions); (x) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:73 (or SEQ ID NO:73 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:74 (or SEQ ID NO:74 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:75 (or SEQ ID NO:75 with one, two, or three amino acid additions, deletions, or substitutions); (xi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:81 (or SEQ ID NO:81 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:82 (or SEQ ID NO:82 with one, two, or three amino acid additions, deletions, or substitutions), Attorney Docket No.48881-0047WO1 / 05958 and SEQ ID NO:83 (or SEQ ID NO:83 with one, two, or three amino acid additions, deletions, or substitutions); (xii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:89 (or SEQ ID NO:89 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:90 (or SEQ ID NO:90 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:91 (or SEQ ID NO:91 with one, two, or three amino acid additions, deletions, or substitutions); (xiii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:97 (or SEQ ID NO:97 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:98 (or SEQ ID NO:98 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:99 (or SEQ ID NO:99 with one, two, or three amino acid additions, deletions, or substitutions); or (xiv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:105 (or SEQ ID NO:105 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:106 (or SEQ ID NO:106 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:107 (or SEQ ID NO:107 with one, two, or three amino acid additions, deletions, or substitutions). The antibody domain can comprise the ability to bind to SEQ ID NO:113 and / or SEQ ID NO:115. The antibody domain can comprise the heavy chain variable domain or region of (i). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:8. The antibody domain can comprise the heavy chain variable domain or region of (ii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:16. The antibody domain can comprise the heavy chain variable domain or region of (iii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:24. The antibody domain can comprise the heavy chain variable domain or region of (iv). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:32. The antibody domain can comprise the heavy chain variable domain or region of (v). The Attorney Docket No.48881-0047WO1 / 05958 heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:40. The antibody domain can comprise the heavy chain variable domain or region of (vi). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:48. The antibody domain can comprise the heavy chain variable domain or region of (vii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:56. The antibody domain can comprise the heavy chain variable domain or region of (viii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:64. The antibody domain can comprise the heavy chain variable domain or region of (ix). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:72. The antibody domain can comprise the heavy chain variable domain or region of (x). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:80. The antibody domain can comprise the heavy chain variable domain or region of (xi). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:88. The antibody domain can comprise the heavy chain variable domain or region of (xii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:96. The antibody domain can comprise the heavy chain variable domain or region of (xiii). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:104. The antibody domain can comprise the heavy chain variable domain or region of (xiv). The heavy chain variable domain or region can comprise an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:112. The Attorney Docket No.48881-0047WO1 / 05958 antibody domain can be monoclonal. The antibody domain can be a VH domain. This paragraph can be referred to as third aspect paragraph. In another aspect, this document features a chimeric antigen receptor comprising (or consisting essentially of, or consisting of) an antigen binding domain, a hinge, a transmembrane domain, and one or more signaling domains, wherein the antigen binding domain comprises (or consists essentially of, or consists of) any antibody, any antigen- binding fragment, or any antibody domain of any of first aspect paragraph, second aspect paragraph, or third aspect paragraph. The antigen binding domain can comprise a VH domain having the ability to bind to a follistatin polypeptide. The hinge can comprise a hinge set forth in Example 23. The transmembrane domain can comprise a transmembrane domain set forth in Example 24. The chimeric antigen receptor can comprise one or more signaling domains set forth in Example 25. This paragraph can be referred to as fourth aspect paragraph. In another aspect, this document features a cell comprising (or consisting essentially of, or consisting of) any chimeric antigen receptor of the preceding paragraph. The cell can be a T cell, a stem cell, or an NK cell. In another aspect, this document features a cell engager comprising (or consisting essentially of, or consisting of) a first antigen binding domain, a linker, and a second antigen binding domain, wherein the first antigen binding domain comprises (or consists essentially of, or consists of) any antibody, any antigen-binding fragment, or any antibody domain of any of first aspect paragraph, second aspect paragraph, or third aspect paragraph. The first antigen binding domain can comprise a VH domain having the ability to bind to a follistatin polypeptide. The first antigen binding domain can be an IgG having the ability to bind to a follistatin polypeptide. The linker can comprise a linker set forth in Example 20 or Example 23. The second antigen binding domain can bind to a polypeptide expressed on the surface of T cells. The polypeptide expressed on the surface of T cells can be a CD3 polypeptide. The second antigen binding domain can be an antigen binding domain set forth in Example 28. The second antigen binding domain can bind to a polypeptide expressed on the surface of NK cells. The polypeptide expressed on the surface of NK cells can be a CD16a, NKG2A, NKG2D, NKp30, Attorney Docket No.48881-0047WO1 / 05958 NKp44, or NKp46 polypeptide. The second antigen binding domain can be an antigen binding domain set forth in Example 29. The cell engager can comprise a third antigen binding domain. The third antigen binding domain can bind to a polypeptide expressed on the surface of NK cells. The polypeptide expressed on the surface of NK cells can be a CD16a, NKG2A, NKG2D, NKp30, NKp44, or NKp46 polypeptide. The third antigen binding domain can be an antigen binding domain set forth in Example 29. This paragraph can be referred to as fifth aspect paragraph. In another aspect, this document features a nucleic acid comprising (or consisting essentially of, or consisting of) a nucleic acid sequence encoding at least part of the antibody, the antigen-binding fragment, or the antibody domain of any of first aspect paragraph, second aspect paragraph, or third aspect paragraph. The nucleic acid sequence can encode the heavy chain variable domain or region of any one of the (i)- (xiv) of first aspect paragraph. The nucleic acid can be a viral vector. The nucleic acid can be a phagemid. In another aspect, this document features a nucleic acid comprising (or consisting essentially of, or consisting of) a nucleic acid sequence encoding any chimeric antigen receptor of fourth aspect paragraph or any cell engager of fifth aspect paragraph. The nucleic acid can be a viral vector. The nucleic acid can be a phagemid. In another aspect, this document features a host cell comprising (or consisting essentially of, or consisting of) any nucleic acid of the preceding paragraph. This paragraph can be referred to as sixth aspect paragraph. In another aspect, this document features a host cell that expresses any chimeric antigen receptor of fourth aspect paragraph or any cell engager of fifth aspect paragraph. The host cell can be a T cell, stem cell, or NK cell. This paragraph can be referred to as seventh aspect paragraph. In another aspect, this document features an antibody-drug conjugate (ADC) comprising (or consisting essentially of, or consisting of) an antigen binging domain covalently linked to a drug, wherein the antigen binging domain comprises an antibody, an antigen binding fragment, or an antibody domain of any of first aspect paragraph, second aspect paragraph, or third aspect paragraph. The antigen binding domain can Attorney Docket No.48881-0047WO1 / 05958 comprise a VH domain having the ability to bind to a follistatin polypeptide. The drug can be selected from the group consisting of auristatins, mertansine, or pyrrolobenzodiazepine (PBD) dimers. This paragraph can be referred to as eighth aspect paragraph. In another aspect, this document features a composition comprising (or consisting essentially of, or consisting of) an antibody, an antigen binding fragment, or an antibody domain of any of first aspect paragraph, second aspect paragraph, or third aspect paragraph. The composition can comprise any antibody of first aspect paragraph. The composition can comprise any antigen binding fragment of second aspect paragraph. The composition can comprise any antibody domain of third aspect paragraph. In another aspect, this document features a composition comprising (or consisting essentially of, or consisting of) any cell engager of fifth aspect paragraph. In another aspect, this document features a composition comprising (or consisting essentially of, or consisting of) any cell of sixth aspect paragraph or seventh aspect paragraph. In another aspect, this document features a composition comprising (or consisting essentially of, or consisting of) any ADC of any one of eighth aspect paragraph. The composition can comprise a checkpoint inhibitor. The checkpoint inhibitor can be selected from the group consisting of cemiplimab, nivolumab, pembrolizumab, JTX- 4014, spartalizumab, camrelizumab, sintilimab, tislelizumab, toripalimab, dostarlimab, INCMGA00012, AMP-224, AMP-514, avelumab, durvalumab, atezolizumab, KN035, CK-301, AUNP12, CA-170, BMS-986189, and ipilimumab. In another aspect, this document features a method of treating a mammal having cancer. The method comprises (or consists essentially of, or consists of) administering, to the mammal, a composition of any of the preceding four paragraphs. The mammal can be a human. The cancer can be a follistatin+cancer. The follistatin+cancer can be selected from the group consisting of follistatin+ovarian cancer, follistatin+lung cancer, and follistatin+prostate cancer. The number of cancer cells within the mammal can be reduced following the administering step. Attorney Docket No.48881-0047WO1 / 05958 In another aspect, this document features a method of treating a mammal having cancer. The method comprises (or consists essentially of, or consists of) (a) administering, to the mammal, a composition of any of the preceding four paragraphs referred to in the preceding paragraph, and (b) administering, to the mammal, a composition comprising a checkpoint inhibitor. The mammal can be a human. The cancer can be a follistatin+cancer. The follistatin+cancer can be selected from the group consisting of follistatin+ovarian cancer, follistatin+lung cancer, and follistatin+prostate cancer. The checkpoint inhibitor can be selected from the group consisting of cemiplimab, nivolumab, pembrolizumab, JTX-4014, spartalizumab, camrelizumab, sintilimab, tislelizumab, toripalimab, dostarlimab, INCMGA00012, AMP-224, AMP- 514, avelumab, durvalumab, atezolizumab, KN035, CK-301, AUNP12, CA-170, BMS- 986189, and ipilimumab. The number of cancer cells within the mammal can be reduced following the administering steps (a) and (b). In another aspect, this document features a method for binding a binding molecule to a follistatin polypeptide. The method comprises (or consists essentially of, or consists of) contacting the follistatin polypeptide with an antibody, an antigen binding fragment, or an antibody domain of any of first aspect paragraph, second aspect paragraph, or third aspect paragraph. The contacting can be performed in vitro. The contacting can be performed in vivo. The contacting can be performed within a mammal by administering the antibody, the antigen binding fragment, or the antibody domain to the mammal. The mammal can be a human. In another aspect, this document features a method for binding a binding molecule to a follistatin polypeptide. The method comprises (or consists essentially of, or consists of) contacting the follistatin polypeptide with any chimeric antigen receptor of fourth aspect paragraph, any cell engager of fifth aspect paragraph, or any ADC of eighth aspect paragraph. The contacting can be performed in vitro. The contacting can be performed in vivo. The contacting can be performed within a mammal by administering the chimeric antigen receptor, the cell engager, or the ADC to the mammal. The mammal can be a human. Attorney Docket No.48881-0047WO1 / 05958 Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. Methods and materials are described herein for use in the present disclosure; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, sequences, database entries, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. DESCRIPTION OF DRAWINGS Figure 1 is a schematic of an exemplary CAR construct designed to express a CAR. A promotor sequence (e.g., a CMV immediate early promotor sequence) can be followed by a signal peptide sequence (e.g., a GM-CSF signal peptide sequence), followed by a scFv provided herein, followed by an optional linker (not shown), followed by an optional hinge (e.g., a CD8 hinge sequence; not shown), followed by a transmembrane sequence (e.g., a CD8 transmembrane sequence), followed by one or more intracellular signaling domain sequences (e.g., a 4-1BB (CD137) intracellular signaling domain sequence and a CD3? intracellular signaling domain sequence). Figure 2 is a schematic of an exemplary CAR construct designed to express a CAR. A promotor sequence (e.g., a CMV immediate early promotor sequence) can be followed by a signal peptide sequence (e.g., a GM-CSF signal peptide sequence), followed by a VH domain provided herein (e.g., a VH domain designed to include a set of three CDRs such as CDR1, CDR2, and CDR3 of a VH domain provided herein, for example, SEQ ID NOs:1-3; SEQ ID NOs:9-11; SEQ ID NOs:17-19; SEQ ID NOs:25-27; SEQ ID NOs:33-35; SEQ ID NOs:41-43; SEQ ID NOs:49-51; SEQ ID NOs:57-59; SEQ ID NOs:65-67; SEQ ID NOs:73-75; SEQ ID NOs:81-83; SEQ ID NOs:89-91; SEQ ID Attorney Docket No.48881-0047WO1 / 05958 NOs:97-99; or SEQ ID NOs:105-107), followed by an optional linker (not shown), followed by an optional hinge (e.g., a CD8 hinge sequence; not shown), followed by a transmembrane sequence (e.g., a CD8 transmembrane sequence), followed by one or more intracellular signaling domain sequences (e.g., a 4-1BB (CD137) intracellular signaling domain sequence and a CD3? intracellular signaling domain sequence). Figure 3 is a schematic of an exemplary BiTE designed using CDR1, CDR2, and CDR3 of a heavy chain and CDR1, CDR2, and CDR3 of a light chain in an Ig format (e.g., an IgG1 format). A humanized anti-CD3 scFv (e.g., an gOKT3-7 scFv set forth in U.S. Patent No.6,750,325) can be linked to the C-terminus of the light chain via a linker (e.g., a (G4S)3 linker; SEQ ID NO:139). Figure 4 is a graph plotting the binding of the indicated VH domain binders to a recombinant human follistatin polypeptide as determined by ELISA. 200 ng of a recombinant human follistatin polypeptide was coated onto immune-absorb plates, and the indicated concentrations of the indicated binders were added. After incubation and washing, horseradish peroxidase (HRP)-conjugated mouse anti-FLAG tag antibody (A8592, Sigma–Aldrich) was used to detect VH binding. Figure 5 is a graph plotting the binding of the indicated binders in an Fc format to a recombinant human follistatin polypeptide as determined by ELISA. 200 ng of a recombinant human follistatin polypeptide was coated onto immune-absorb plates, and the indicated concentrations of the indicated binders were added. After incubation and washing, HRP-conjugated goat anti-human IgG Fc (Sigma-Aldrich) was used for detection of VH-Fc binding. Since FST1E8, FST2D4, and FST3B3 are very close to each other in sequence, only FST1E8 was transferred to the VH-Fc format and used for this ELISA. Figure 6 is a graph plotting the binding of the indicated VH domain binders to BSA as determined by ELISA. 200 ng of BSA was coated onto immune-absorb plates, and the indicated concentrations of the indicated binders were added. After incubation and washing, horseradish peroxidase (HRP)-conjugated mouse anti-FLAG tag antibody (A8592, Sigma–Aldrich) was used to detect VH binding. Attorney Docket No.48881-0047WO1 / 05958 Figure 7 contains graphs of Propidium Iodide (PI) and Annexin-V flow analysis of primary ovarian cancer cells treated with the indicated antibodies or anti-follistatin VH Clones in the presence or absence of cisplatin. Viable cells are in the lower left quadrant. The results show that the IgG alone or anti-follistatin VH Clones alone do not significantly impact ovarian cancer cell viability. However, follistatin neutralization with either tested anti-follistatin VH clone (i.e., FST3G9 or FST1E8) significantly enhanced cell killing of cisplatin. Figure 8 contains graphs plotting nuclear DNA content in ovarian cancer cells treated with or without chemotherapy (taxol) in the presence or absence of an anti- follistatin VH clone (FST3G9). The results show that taxol increases nuclear content leading to mitotic catastrophe. Addition of an anti-follistatin VH clone (FST3G9) significantly enhanced this effect. P3 = 1N nuclear content; P4 = 2N nuclear content; P5 = 3N nuclear content; and P6 > 3N nuclear content). Figure 9 contains photographs of immunofluorescent imaging of F-actin (which appeared as red) and DAPI (which appeared as blue) in ovarian cancer cells with the indicated treatments. FST-Ab = anti-follistatin VH Clone FST3G9. The results show that taxol treated cells increase in size and become multinucleate with mitotic catastrophe. Addition of an anti-follistatin VH clone (FST3G9) to taxol treatment resulted in fewer cells, and only large multinucleate cells in mitotic crisis persisted. Figure 10 includes three graphs plotting the results of competitive biolayer interferometry BLItz assays used to map different binding regions of binders in follistatin. Based on these results, binders with five different binding regions in follistatin were separated and identified. Figure 11 includes graphs plotting the results of a chemotherapy synergy study using Propidium Iodide (PI) and Annexin-V flow analysis of primary ovarian cancer cells treated with vehicle, cisplatin (Cis), or Cis in combination with various anti-follistatin VH Clones. Viable cells are shown in the lower left quadrant of each graph. The final VH concentration in each treatment group was the same. Attorney Docket No.48881-0047WO1 / 05958 DETAILED DESCRIPTION This document provides binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, cell engagers, and ADCs) that bind (e.g., specifically bind) to a follistatin polypeptide (e.g., a human follistatin polypeptide). For example, the document provides binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, cell engagers, and ADCs) that bind (e.g., specifically bind) to a polypeptide comprising, consisting essentially of, or consisting of the amino acid set forth in SEQ ID NO:113 and / or SEQ ID NO:115 (see, e.g., Example 2). The term “antibody” as used herein includes polyclonal antibodies, monoclonal antibodies, recombinant antibodies, humanized antibodies, human antibodies, chimeric antibodies, multi-specific antibodies (e.g., bispecific antibodies) formed from at least two antibodies, diabodies, single-chain variable fragment antibodies (e.g., scFv antibodies), and tandem single-chain variable fragments antibody (e.g., taFv). A diabody can include two chains, each having a heavy chain variable domain and a light chain variable domain, either from the same or from different antibodies (see, e.g., Hornig and Färber-Schwarz, Methods Mol. Biol., 907:713-27 (2012); and Brinkmann and Kontermann, MAbs., 9(2):182-212 (2017)). The two variable regions can be connected by a polypeptide linker (e.g., a polypeptide linker having five to ten residues in length or a polypeptide linker as set forth in Example 20). In some cases, an interdomain disulfide bond can be present in one or both of the heavy chain variable domain and light chain variable domain pairs of the diabody. A scFv is a single-chain polypeptide antibody in which the heavy chain variable domain and the light chain variable domain are directly connected or connected via a polypeptide linker (e.g., a polypeptide linker having eight to 18 residues in length or a polypeptide linker as set forth in Example 20). See, also, Chen et al., Adv. Drug Deliv. Rev., 65(10):1357-1369 (2013). A scFv can be designed to have an orientation with the heavy chain variable domain being followed by the light chain variable domain or can be designed to have an orientation with the light chain variable domain being followed by the heavy chain variable domain. In both cases, the optional linker can be located between the two domains. Examples of scFv structures of scFvs provided herein include, without limitation, those structures set forth in Example 19. Attorney Docket No.48881-0047WO1 / 05958 An antibody provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be configured to be a human antibody, a humanized antibody, or a chimeric antibody. In some cases, an antibody provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be a monoclonal antibody. In some cases, an antibody provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be configured as a scFv antibody. The term “antigen binding fragment” as used herein refers to a fragment of an antibody (e.g., a fragment of a humanized antibody, a fragment of a human antibody, or a fragment of a chimeric antibody) having the ability to bind to an antigen. Examples of antigen binding fragments include, without limitation, Fab, Fab’, or F(ab’)2 antigen binding fragments. An antigen binding fragment provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be configured to be a human antigen binding fragment, a humanized antigen binding fragment, or a chimeric antigen binding fragment. In some cases, an antigen binding fragment provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be a monoclonal antigen binding fragment. In some cases, an antigen binding fragment provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be configured as a Fab antibody. In some cases, a Fab antibody can include a partial hinge sequence (e.g., EPKSCDKT (SEQ ID NO:228)) for disulfide bonding between heavy and light chains of the Fab. The term “antibody domain” as used herein refers to a domain of an antibody such as a heavy chain variable domain (VH domain) or a light chain variable domain (VL domain) in the absence of one or more other domains of an antibody. In some cases, an antibody domain can be a single antibody domain (e.g., a VH domain or a VL domain) having the ability to bind to an antigen. An antibody domain provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be a human antibody domain (e.g., a human VH domain), a humanized antibody domain (e.g., a humanized VH domain), or a chimeric antibody domain (e.g., a chimeric VH domain). In some cases, an antibody domain provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be a monoclonal antibody domain. In Attorney Docket No.48881-0047WO1 / 05958 some cases, an antibody domain provided herein can include the CDRs as described herein (e.g., as described in Table 43) and can be engineered as a single VH domain or a single VL domain. Examples of VH domains provided herein include, without limitation, those structures set forth in Examples 3-16. An anti-follistatin antibody, anti-follistatin antigen binding fragment, or anti- follistatin antibody domain provided herein can be of the IgA-, IgD-, IgE-, IgG-, or IgM- type, including IgG- or IgM-types such as, without limitation, IgG1-, IgG2-, IgG3-, IgG4-, IgM1-, and IgM2-types. In some cases, an antibody provided herein (e.g., an anti- follistatin antibody) can be a scFv antibody. In some cases, an antigen binding fragment provided herein (e.g., an anti-follistatin antibody fragment) can be a Fab. In some cases, an antibody provided herein (e.g., an anti-follistatin antibody) can be a fully intact antibody having the structure set forth in Example 18. In some cases, an antibody domain provided herein (e.g., an anti-follistatin antibody domain) can be a VH domain. In some cases, an anti-follistatin antibody, anti-follistatin antigen binding fragment, or anti-follistatin antibody domain provided herein can be fully human. In some cases, an anti-follistatin antibody, anti-follistatin antigen binding fragment, or anti- follistatin antibody domain provided herein can have a low risk for inducing immunogenicity within a human. The term “chimeric antigen receptor” as used herein refers to a chimeric polypeptide that is designed to include an optional signal peptide, an antigen binding domain, an optional hinge, a transmembrane domain, and one or more intracellular signaling domains. As described herein, the antigen binding domain of a CAR provided herein can be designed to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). For example, a CAR provided herein can be designed to include the components of an antibody, antigen binding fragment, and / or antibody domain described herein (e.g., a combination of CDRs) as an antigen binding domain provided that that antigen binding domain has the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). In some examples, a CAR provided herein can be designed to include an antigen binding domain that includes a single set of three CDRs (e.g., a CDR1, CDR2, and CDR3) of an antibody domain (e.g., a VH domain) provided herein (e.g., Attorney Docket No.48881-0047WO1 / 05958 SEQ ID NOs:1-3; SEQ ID NOs:9-11; SEQ ID NOs:17-19; SEQ ID NOs:25-27; SEQ ID NOs:33-35; SEQ ID NOs:41-43; SEQ ID NOs:49-51; SEQ ID NOs:57-59; SEQ ID NOs:65-67; SEQ ID NOs:73-75; SEQ ID NOs:81-83; SEQ ID NOs:89-91; SEQ ID NOs:97-99; or SEQ ID NOs:105-107). In some cases, an antigen binding domain of a CAR targeting a follistatin polypeptide can be designed to include a VH domain described herein or a scFv antibody described herein. Examples of CAR structures that can be used to make a CAR provided herein include, without limitation, those set forth in Figures 1 and 2 and Examples 21 and 26. In some cases, a CAR provided herein can be designed to include a signal peptide. Any appropriate signal peptide can be used to design a CAR described herein. Examples of signal peptide that can be used to make a CAR described herein include without limitation, a human IGKV1-39-derived signal peptide, IGKV1-16, IGKV1-33, IGKV3- 11, IGKV4-1, or IGKV6-21. In some cases, a CAR provided herein can be designed to include a signal peptide that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 22. In some cases, a CAR provided herein can be designed to include a signal peptide that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 22 with one, two, three, four, five, six, seven, eight, nine, or ten amino acid deletions, additions, substitutions, or combinations thereof. In some cases, a CAR provided herein can be designed to include a signal peptide that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 22 with two or less, three or less, four or less, five or less, six or less, seven or less, eight or less, nine or less, or ten or less amino acid deletions, additions, substitutions, or combinations thereof. In some cases, a CAR provided herein can be designed to include a hinge. Any appropriate hinge can be used to design a CAR described herein. Examples of hinges that can be used to make a CAR described herein include, without limitation, Ig-derived hinges (e.g., an IgG1-derived hinge, an IgG2-derived hinge, or an IgG4-derived hinge), Ig-derived hinges containing a CD2 domain and a CD3 domain, Ig-derived hinges containing a CD2 domain and lacking a CD3 domain, Ig-derived hinges containing a CD3 domain and lacking a CD2 domain, Ig-derived hinges lacking a CD2 domain and Attorney Docket No.48881-0047WO1 / 05958 lacking a CD3 domain, CD8?-derived hinges, CD28-derived hinges, and CD3?-derived hinges. A CAR provided herein can be designed to include a hinge of any appropriate length. For example, a CAR provided herein can be designed to include a hinge that is from about 3 to about 75 (e.g., from about 3 to about 65, from about 3 to about 50, from about 5 to about 75, from about 10 to about 75, from about 5 to about 50, from about 10 to about 50, from about 10 to about 40, or from about 10 to about 30) amino acid residues in length. In some cases, a linker sequence can be used as a hinge to make a CAR described herein. For example, any one of the linker sequences set forth in Example 20 can be used as a hinge of a CAR described herein. In some cases, a CAR provided herein can be designed to include a hinge that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 20 or Example 23. In some cases, a CAR provided herein can be designed to include a hinge that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 20 or Example 23 with one, two, three, four, five, six, seven, eight, nine, or ten amino acid deletions, additions, substitutions, or combinations thereof. In some cases, a CAR provided herein can be designed to include a hinge that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 20 or Example 23 with two or less, three or less, four or less, five or less, six or less, seven or less, eight or less, nine or less, or ten or less amino acid deletions, additions, substitutions, or combinations thereof. A CAR provided herein can be designed to include any appropriate transmembrane domain. For example, the transmembrane domain of a CAR provided herein can be, without limitation, a CD3? transmembrane domain, a CD4 transmembrane domain, a CD8? transmembrane domain, a CD28 transmembrane domain, and a 4-1BB transmembrane domain. In some cases, a CAR provided herein can be designed to include a transmembrane domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 24. In some cases, a CAR provided herein can be designed to include a transmembrane domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 24 with one, two, three, four, five, six, seven, eight, nine, or ten amino acid deletions, Attorney Docket No.48881-0047WO1 / 05958 additions, substitutions, or combinations thereof. In some cases, a CAR provided herein can be designed to include a transmembrane domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 24 with two or less, three or less, four or less, five or less, six or less, seven or less, eight or less, nine or less, or ten or less amino acid deletions, additions, substitutions, or combinations thereof. A CAR provided herein can be designed to include one or more intracellular signaling domains. For example, a CAR provided herein can be designed to include one, two, three, or four intracellular signaling domains. Any appropriate intracellular signaling domain or combination of intracellular signaling domains can be used to make a CAR described herein. Examples of intracellular signaling domains that can be used to make a CAR described herein include, without limitation, CD3? intracellular signaling domains, CD27 intracellular signaling domains, CD28 intracellular signaling domains, OX40 (CD134) intracellular signaling domains, 4-1BB (CD137) intracellular signaling domains, CD278 intracellular signaling domains, DAP10 intracellular signaling domains, and DAP12 intracellular signaling domains. In some cases, a CAR described herein can be designed to be a first generation CAR having a CD3? intracellular signaling domain. In some cases, a CAR described herein can be designed to be a second generation CAR having a CD28 intracellular signaling domain followed by a CD3? intracellular signaling domain. In some cases, a CAR described herein can be designed to be a third generation CAR having (a) a CD28 intracellular signaling domain followed by (b) a CD27 intracellular signaling domain, an OX40 intracellular signaling domains, or a 4-1BB intracellular signaling domain followed by (c) a CD3? intracellular signaling domain. In some cases, a CAR provided herein can be designed to include at least one intracellular signaling domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 25. In some cases, a CAR provided herein can be designed to include at least one intracellular signaling domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 25 with one, two, three, four, five, six, seven, eight, nine, or ten amino acid deletions, additions, substitutions, or combinations thereof, provided that that intracellular signaling domain has at least some activity to activate intracellular signaling. In some cases, a Attorney Docket No.48881-0047WO1 / 05958 CAR provided herein can be designed to include at least one intracellular signaling domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 25 with two or less, three or less, four or less, five or less, six or less, seven or less, eight or less, nine or less, or ten or less amino acid deletions, additions, substitutions, or combinations thereof, provided that that intracellular signaling domain has at least some activity to activate intracellular signaling. In some cases, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:57, SEQ ID NO:58, and SEQ ID NO:59, followed by a hinge such as a hinge / linker set forth in Example 20 or Example 23 (e.g., an IgG4-derived hinge, a CD8? hinge, or a linker plus IgG4-derived hinge), followed by a transmembrane domain such as a transmembrane domain set forth in Example 24 (e.g., a human CD28 transmembrane domain or a CD8? transmembrane domain), followed by one or more intracellular signaling domains such as one or more intracellular signaling domain set forth in Example 25 (e.g., a human 4-1BB intracellular signaling domain followed by a human CD3? intracellular signaling domain). For example, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:57, SEQ ID NO:58, and SEQ ID NO:59, followed by SEQ ID NO:178, followed by SEQ ID NO:184, followed by SEQ ID NO:192, followed by SEQ ID NO:191 (see, e.g., Example 26). In another example, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:57, SEQ ID NO:58, and SEQ ID NO:59, followed by SEQ ID NO:180, followed by SEQ ID NO:187, followed by SEQ ID NO:194, followed by SEQ ID NO:191 (see, e.g., Example 26). In some cases, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:64, followed by a hinge such as a hinge / linker set forth in Example 20 or Example 23 (e.g., an IgG4-derived hinge, a CD8? hinge, or a linker plus IgG4-derived hinge), followed by a transmembrane domain such as a transmembrane domain set forth in Example 24 (e.g., a human CD28 transmembrane domain or a CD8? transmembrane domain), followed by one or more intracellular signaling domains such as one or more intracellular signaling domain set forth in Attorney Docket No.48881-0047WO1 / 05958 Example 25 (e.g., a human 4-1BB intracellular signaling domain followed by a human CD3? intracellular signaling domain). For example, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:64, followed by SEQ ID NO:178, followed by SEQ ID NO:184, followed by SEQ ID NO:192, followed by SEQ ID NO:191 (see, e.g., Example 26). In another example, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:64, followed by SEQ ID NO:180, followed by SEQ ID NO:187, followed by SEQ ID NO:194, followed by SEQ ID NO:191 (see, e.g., Example 26). In some cases, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:65, SEQ ID NO:66, and SEQ ID NO:67, followed by a hinge such as a hinge / linker set forth in Example 20 or Example 23 (e.g., an IgG4-derived hinge, a CD8? hinge, or a linker plus IgG4-derived hinge), followed by a transmembrane domain such as a transmembrane domain set forth in Example 24 (e.g., a human CD28 transmembrane domain or a CD8? transmembrane domain), followed by one or more intracellular signaling domains such as one or more intracellular signaling domain set forth in Example 25 (e.g., a human 4-1BB intracellular signaling domain followed by a human CD3? intracellular signaling domain). For example, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:65, SEQ ID NO:66, and SEQ ID NO:67, followed by SEQ ID NO:178, followed by SEQ ID NO:184, followed by SEQ ID NO:192, followed by SEQ ID NO:191 (see, e.g., Example 26). In another example, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:65, SEQ ID NO:66, and SEQ ID NO:67, followed by SEQ ID NO:180, followed by SEQ ID NO:187, followed by SEQ ID NO:194, followed by SEQ ID NO:191 (see, e.g., Example 26). In some cases, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:72, followed by a hinge such as a hinge / linker set forth in Example 20 or Example 23 (e.g., an IgG4-derived hinge, a CD8? hinge, or a linker plus IgG4-derived hinge), followed by a transmembrane domain such as a transmembrane domain set forth in Example 24 (e.g., a human CD28 transmembrane Attorney Docket No.48881-0047WO1 / 05958 domain or a CD8? transmembrane domain), followed by one or more intracellular signaling domains such as one or more intracellular signaling domain set forth in Example 25 (e.g., a human 4-1BB intracellular signaling domain followed by a human CD3? intracellular signaling domain). For example, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:72, followed by SEQ ID NO:178, followed by SEQ ID NO:184, followed by SEQ ID NO:192, followed by SEQ ID NO:191 (see, e.g., Example 26). In another example, a CAR targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:72, followed by SEQ ID NO:180, followed by SEQ ID NO:187, followed by SEQ ID NO:194, followed by SEQ ID NO:191 (see, e.g., Example 26). The term “cell engager” as used herein refers to a polypeptide that includes two or more antigen binding domains (e.g., two, three, or four antigen binding domains) and has the ability to link two cells together. Examples of cell engagers include, without limitation, BiTEs, BiKEs, and TriKEs. In general, a cell engager provided herein can be designed to include at least one antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and at least one antigen binding domain having the ability to bind to an antigen expressed on the surface of a cell (e.g., a T cell or an NK cell). In some cases, a cell engager described herein can link a follistatin+cell (e.g., a follistatin+cancer cell) to another cell (e.g., a T cell or an NK cell) via the two or more antigen binding domains of the cell engager. An example of a cell engager structure of cell engagers provided herein includes, without limitation, the structure set forth in Figure 3. In some cases, the anti-CD3 scFv depicted in Figure 3 can be replaced with a different antigen binding domain having the ability to bind to an antigen expressed on the surface of a cell (e.g., a T cell or an NK cell). When a cell engager includes an antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and two or more other antigen binding domains (e.g., two, three, or four other antigen binding domains), each of those other antigen binding domains can bind to different antigens expressed on the surface of different cell types or can bind to different antigens expressed on the surface of the same cell type. For example, a TriKE can be designed to have a first Attorney Docket No.48881-0047WO1 / 05958 antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide), a second antigen binding domain having the ability to bind to a first antigen expressed on the surface of an NK cell (e.g., a CD16 polypeptide such as a CD16a polypeptide), and a third antigen binding domain having the ability to bind to a second antigen expressed on the surface of an NK cell (e.g., an NKG2A polypeptide). As described herein, at least one antigen binding domain of a cell engager provided herein can be designed to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). For example, a cell engager provided herein can be designed to include the components of an antibody, antigen binding fragment, and / or antibody domain described herein (e.g., a combination of CDRs) as an antigen binding domain provided that that antigen binding domain has the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). In some examples, a cell engager provided herein can be designed to include an antigen binding domain that includes two sets of three CDRs (e.g., CDR1, CDR2, and CDR3 of a heavy chain and CDR1, CDR2, and CDR3 of a light chain) of an antigen binding fragment. In some examples, a cell engager provided herein can be designed to include an antigen binding domain that includes a single set of three CDRs (e.g., a CDR1, CDR2, and CDR3) of an antibody domain (e.g., a VH domain) provided herein (e.g., SEQ ID NOs:1-3; SEQ ID NOs:9-11; SEQ ID NOs:17-19; SEQ ID NOs:25-27; SEQ ID NOs:33- 35; SEQ ID NOs:41-43; SEQ ID NOs:49-51; SEQ ID NOs:57-59; SEQ ID NOs:65-67; SEQ ID NOs:73-75; SEQ ID NOs:81-83; SEQ ID NOs:89-91; SEQ ID NOs:97-99; or SEQ ID NOs:105-107). In some cases, an antigen binding domain of a cell engager targeting a follistatin polypeptide can be designed to include a VH domain described herein or a scFv / Fab antibody described herein. In some cases, an antigen binding domain of a CAR described herein that has the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can be used as an antigen binding domain of a cell engager that targets follistatin+cells. Attorney Docket No.48881-0047WO1 / 05958 As described herein, a cell engager can be designed to include at least one antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and at least one other antigen binding domain. That at least one other antigen binding domain can have the ability to bind to any appropriate antigen expressed on the surface of a cell. For example, when designing a cell engager such as a BiTE to link a follistatin+cell and a T cell, the cell engager can include an antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and an antigen binding domain having the ability to bind to a polypeptide expressed on the surface of a T cell. Examples of polypeptides expressed on the surface of a T cell that can be targeted by an antigen binding domain of a cell engager provided herein include, without limitation, CD3 polypeptides. Examples of antigen binding domains having the ability to bind to a polypeptide expressed on the surface of a T cell that can be used to make a cell engager provided herein (e.g., a BiTE) include, without limitation, anti-CD3 scFvs and anti-CD3 VH domains. Additional examples of amino acid sequences that can be used as antigen binding domains having the ability to bind to a polypeptide expressed on the surface of a T cell (e.g., CD3) are described in U.S. Patent No.6,750,325 (see, e.g., the sequence listing of U.S. Patent No.6,750,325). Examples of BiTE structures that can be used to make a BiTE provided herein include, without limitation, those set forth in Examples 30 and 31. In some cases, a cell engager provided herein can be designed to include an antigen binding domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 28. In some cases, a cell engager provided herein can be designed to include an antigen binding domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 28 with one, two, three, four, five, six, seven, eight, nine, or ten amino acid deletions, additions, substitutions, or combinations thereof, provided that the antigen binding domain has the ability to bind to a polypeptide expressed on the surface of a T cell. In some cases, a cell engager provided herein can be designed to include an antigen binding domain that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 28 with two or less, three or less, four or less, five or less, Attorney Docket No.48881-0047WO1 / 05958 six or less, seven or less, eight or less, nine or less, or ten or less amino acid deletions, additions, substitutions, or combinations thereof, provided that the antigen binding domain has the ability to bind to a polypeptide expressed on the surface of a T cell. When designing a cell engager such as a BiKE or a TriKE to link a follistatin+cell and an NK cell, the cell engager can include an antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and one or more (e.g., one, two, or three) antigen binding domains having the ability to bind to a polypeptide expressed on the surface of an NK cell. Examples of polypeptides expressed on the surface of an NK cell that can be targeted by an antigen binding domain of a cell engager provided herein include, without limitation, CD16 polypeptides (e.g., CD16a polypeptides), NKG2A polypeptides, NKG2D polypeptides, NKp30 polypeptides, NKp44 polypeptides, and NKp46 polypeptides. Examples of antigen binding domains having the ability to bind to a polypeptide expressed on the surface of an NK cell that can be used to make a cell engager provided herein (e.g., a BiKE or TriKE) include, without limitation, anti-CD16a scFvs, anti-NKG2A scFvs, anti-NKG2D scFvs, anti-NKp30 scFvs (see, e.g., BioLegend Catalog #325207), anti-NKp44 scFvs, anti-NKp46 scFvs, anti- CD16a VH domains, anti-NKG2A VH domains, anti-NKG2D VH domains, anti-NKp30 VH domains, anti-NKp44 VH domains, and anti-NKp46 VH domains. Additional examples of amino acid sequences that can be used as antigen binding domains having the ability to bind to a polypeptide expressed on the surface of an NK cell (e.g., CD16, NKG2A, NKG2D, or NKp46) are described in McCall et al. (Mol. Immunol., 36(7):433- 445 (1999); see, e.g., anti-CD16 scFv sequences); International Patent Application Publication No. PCT / US2017 / 048721 (see, e.g., the CDRs and sequence listing for anti- CD16a binding domains); U.S. Patent Application Publication No.2011 / 0052606 (see, e.g., the CDRs and the sequence listing for anti-NKG2A antibodies such as Z199); U.S. Patent Application Publication No.2011 / 0150870 (see, e.g., the CDRs and sequence listing for anti-NKG2D antibodies); U.S. Patent Application Publication No. 2018 / 0369373 (see, e.g., the CDRs and sequence listing for anti-NKp46 antibodies); and U.S. Patent Application Publication No.2017 / 0368169 (see, e.g., the CDRs and sequence listing for anti-NKp46 antibodies). Attorney Docket No.48881-0047WO1 / 05958 In some cases, a cell engager provided herein can be designed to include an antigen binding domain (e.g., a scFv or VH) that comprises, consists essentially of, or consists of one or more of the amino acid sequences set forth in Example 29. In some cases, a cell engager provided herein can be designed to include an antigen binding domain (e.g., a scFv or VH) that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 29 with one, two, three, four, five, six, seven, eight, nine, or ten amino acid deletions, additions, substitutions, or combinations thereof, provided that the antigen binding domain has the ability to bind to a polypeptide expressed on the surface of an NK cell. In some cases, a cell engager provided herein can be designed to include an antigen binding domain (e.g., a scFv or VH) that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 29 with two or less, three or less, four or less, five or less, six or less, seven or less, eight or less, nine or less, or ten or less amino acid deletions, additions, substitutions, or combinations thereof, provided that the antigen binding domain has the ability to bind to a polypeptide expressed on the surface of an NK cell. In some cases, a cell engager provided herein can be designed to include a linker located between each antigen binding domain. Any appropriate linker can be used to design a cell engager provided herein. Examples of linkers that can be used to make a cell engager described herein include, without limitation, the linker sequences set forth in Example 20. A cell engager provided herein can be designed to include a linker of any appropriate length. For example, a cell engager provided herein can be designed to include a linker that is from about 3 to about 100 (e.g., from about 3 to about 90, from about 3 to about 80, from about 3 to about 70, from about 3 to about 60, from about 3 to about 50, from about 3 to about 40, from about 3 to about 30, from about 3 to about 20, from about 3 to about 15, from about 5 to about 100, from about 10 to about 100, from about 20 to about 100, from about 30 to about 100, from about 40 to about 100, from about 50 to about 100, from about 60 to about 100, from about 70 to about 100, from about 10 to about 50, from about 10 to about 40, from about 10 to about 30, from about 10 to about 20, or from about 12 to about 17) amino acid residues in length. In some cases, a cell engager provided herein (e.g., a BiTE) can be designed to include a Attorney Docket No.48881-0047WO1 / 05958 GGGGSGGGGSGGGGS (SEQ ID NO:139) linker. In some cases, a hinge of a CAR described herein can be used as a linker to make a cell engager described herein. For example, any one of the sequences set forth in Example 23 can be used as a linker of a cell engager described herein. In some cases, a cell engager provided herein can be designed to include a linker that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 20 or Example 23. In some cases, a cell engager provided herein can be designed to include a linker that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 20 or Example 23 with one, two, three, four, five, six, seven, eight, nine, or ten amino acid deletions, additions, substitutions, or combinations thereof. In some cases, a cell engager provided herein can be designed to include a linker that comprises, consists essentially of, or consists of one of the amino acid sequences set forth in Example 20 or Example 23 with two or less, three or less, four or less, five or less, six or less, seven or less, eight or less, nine or less, or ten or less amino acid deletions, additions, substitutions, or combinations thereof. In some cases, a cell engager (e.g., a BiTE) targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:57, SEQ ID NO:58, and SEQ ID NO:59, followed by a linker such as a linker / hinge set forth in Example 20 or Example 23 (e.g., SEQ ID NO:139 or 159), followed by an antigen binding domain having the ability to bind to a polypeptide expressed on the surface of a T cell (e.g., an anti-human CD3 scFv) (see, e.g., Example 30). In some cases, a cell engager (e.g., a BiTE) targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:64, followed by a linker such as a linker / hinge set forth in Example 20 or Example 23 (e.g., SEQ ID NO:139 or 159), followed by an antigen binding domain having the ability to bind to a polypeptide expressed on the surface of a T cell (e.g., an anti-human CD3 scFv) (see, e.g., Example 30). In some cases, a cell engager (e.g., a BiTE) targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:65, SEQ ID NO:66, and SEQ ID NO:67, followed by a linker such as a linker / hinge set forth in Example 20 or Attorney Docket No.48881-0047WO1 / 05958 Example 23 (e.g., SEQ ID NO:139 or 159), followed by an antigen binding domain having the ability to bind to a polypeptide expressed on the surface of a T cell (e.g., an anti-human CD3 scFv) (see, e.g., Example 31). In some cases, a cell engager (e.g., a BiTE) targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:72, followed by a linker such as a linker / hinge set forth in Example 20 or Example 23 (e.g., SEQ ID NO:139 or 159), followed by an antigen binding domain having the ability to bind to a polypeptide expressed on the surface of a T cell (e.g., an anti-human CD3 scFv) (see, e.g., Example 31). In some cases, a cell engager (e.g., a BiKE or a TriKE) targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:57, SEQ ID NO:58, and SEQ ID NO:59, followed by a linker such as a linker / hinge set forth in Example 20 or Example 23 (e.g., SEQ ID NO:139 or 159), followed by one or more antigen binding domains having the ability to bind to a polypeptide expressed on the surface of an NK cell (e.g., an anti-human CD16a scFv for a BiKE or an anti-human CD16a scFv and an anti-human NKG2A scFv for a TriKE). In some cases, a cell engager (e.g., a BiKE or a TriKE) targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:64, followed by a linker such as a hinge / linker set forth in Example 20 or Example 23 (e.g., SEQ ID NO:139 or 159), followed by one or more antigen binding domains having the ability to bind to a polypeptide expressed on the surface of an NK cell (e.g., an anti- human CD16a scFv for a BiKE or an anti-human CD16a scFv and an anti-human NKG2A scFv for a TriKE). In some cases, a cell engager (e.g., a BiKE or a TriKE) targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:65, SEQ ID NO:66, and SEQ ID NO:67, followed by a linker such as a linker / hinge set forth in Example 20 or Example 23 (e.g., SEQ ID NO:139 or 159), followed by one or more antigen binding domains having the ability to bind to a polypeptide expressed on the surface of an NK cell (e.g., an anti-human CD16a scFv for a BiKE or an anti-human CD16a scFv and an anti-human NKG2A scFv for a TriKE). Attorney Docket No.48881-0047WO1 / 05958 In some cases, a cell engager (e.g., a BiKE or a TriKE) targeting a follistatin polypeptide can be designed to include a VH domain comprising SEQ ID NO:72, followed by a linker such as a hinge / linker set forth in Example 20 or Example 23 (e.g., SEQ ID NO:139 or 159), followed by one or more antigen binding domains having the ability to bind to a polypeptide expressed on the surface of an NK cell (e.g., an anti- human CD16a scFv for a BiKE or an anti-human CD16a scFv and an anti-human NKG2A scFv for a TriKE). In one embodiment, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:1 (or a variant of SEQ ID NO:1 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:2 (or a variant of SEQ ID NO:2 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:3 (or a variant of SEQ ID NO:3 with one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 3. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:1 (or a variant of SEQ ID NO:1 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:2 (or a variant of SEQ ID NO:2 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:3 (or a variant of SEQ ID NO:3 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:4 (or a Attorney Docket No.48881-0047WO1 / 05958 variant of SEQ ID NO:4 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:5 (or a variant of SEQ ID NO:5 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:6 (or a variant of SEQ ID NO:6 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:7 (or a variant of SEQ ID NO:7 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 3 can be designed to include framework regions as set forth in Example 3 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 3 and the framework regions set forth in Example 3 except that framework region 1 having the amino acid set forth in SEQ ID NO:4 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:12, a framework region 1 having the amino acid set forth in SEQ ID NO:20, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 3, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:8. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 Attorney Docket No.48881-0047WO1 / 05958 percent identity to the amino acid sequence set forth in SEQ ID NO:8. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:8. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:8, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:1, 2, and 3. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:8, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:1, 2, and 3. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:8 or the amino acid set forth in SEQ ID NO:8 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:8 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:1, 2, and 3. Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:1, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:2, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:3. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:1” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:1, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:1, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:1, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:1 include, without limitation, those set forth in Table 1. Table 1. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:1. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 GGSISSGYY259 et forth in SEQ ID NO:2” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:2, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:2, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:2, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:2 include, without limitation, those set forth in Table 2. Table 2. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:2. Sequence SEQ ID NO: As used herein, a “CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:3” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:3, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:3, and / or that has zero, one, two, three, four, or five amino Attorney Docket No.48881-0047WO1 / 05958 acid residues directly following SEQ ID NO:3, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:3 include, without limitation, those set forth in Table 3. Table 3. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:3. Sequence SEQ ID NO: In another embodiment, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:9 (or a variant of SEQ ID NO:9 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:10 (or a variant of SEQ ID NO:10 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:11 (or a variant of SEQ ID NO:11 with one or two amino acid modifications). An example of such a binder having these CDRs Attorney Docket No.48881-0047WO1 / 05958 and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 4. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:9 (or a variant of SEQ ID NO:9 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:10 (or a variant of SEQ ID NO:10 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:11 (or a variant of SEQ ID NO:11 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:12 (or a variant of SEQ ID NO:12 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:13 (or a variant of SEQ ID NO:13 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:14 (or a variant of SEQ ID NO:14 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:15 (or a variant of SEQ ID NO:15 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 4 can be designed to include framework regions as set forth in Example 4 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 4 and the framework regions set forth in Example 4 except that framework region 1 having the amino acid set forth in SEQ ID Attorney Docket No.48881-0047WO1 / 05958 NO:12 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:20, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 4, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:16. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:16. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:16. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:16, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:9, 10, and 11. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in Attorney Docket No.48881-0047WO1 / 05958 SEQ ID NO:16, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs: 9, 10, and 11. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:16 or the amino acid set forth in SEQ ID NO:16 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:16 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs: 9, 10, and 11. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:9, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:10, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:11. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:9” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:9, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:9, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:9, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists Attorney Docket No.48881-0047WO1 / 05958 essentially of the amino acid sequence set forth in SEQ ID NO:9 include, without limitation, those set forth in Table 4. Table 4. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:9. Sequence SEQ ID NO: 280 As used herein, a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:10” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:10, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:10, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:10, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:10 include, without limitation, those set forth in Table 5. Table 5. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:10. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 INPNSGGT 290 INPSGGST 291 s use ere n, a t at cons sts essent a y o t e amno ac sequence set forth in SEQ ID NO:11” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:11, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:11, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:11, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:11 include, without limitation, those set forth in Table 6. Table 6. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:11. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 ARIGGPIVGALTVPWMVV 305 AKDGALTVPWMEPRVV 306 , . ., , , antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:17 (or a variant of SEQ ID NO:17 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:18 (or a variant of SEQ ID NO:18 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:19 (or a variant of SEQ ID NO:19 with one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 5. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:17 (or a variant of SEQ ID NO:17 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:18 (or a variant of SEQ ID NO:18 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:19 (or a variant of SEQ ID NO:19 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:20 (or a variant of SEQ ID NO:20 with one, two, three, four, five, six, seven, eight, nine, ten, or Attorney Docket No.48881-0047WO1 / 05958 more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:21 (or a variant of SEQ ID NO:21 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:22 (or a variant of SEQ ID NO:22 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:23 (or a variant of SEQ ID NO:23 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 5 can be designed to include framework regions as set forth in Example 5 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 5 and the framework regions set forth in Figure 4 except that framework region 1 having the amino acid set forth in SEQ ID NO:20 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 5, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:24. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:24. In some cases, a Attorney Docket No.48881-0047WO1 / 05958 binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:24. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:24, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:17, 18, and 19. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:24, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:17, 18, and 19. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:24 or the amino acid set forth in SEQ ID NO:24 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:24 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:17, 18, and 19. Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:17, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:18, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:19. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:17” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:17, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:17, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:17, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:17 include, without limitation, those set forth in Table 7. Table 7. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:17. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 GGSISSGYY 319 et forth in SEQ ID NO:18” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:18, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:18, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:18, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:18 include, without limitation, those set forth in Table 8. Table 8. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:18. Sequence SEQ ID NO: As used herein, a “CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:19” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:19, that has zero, one, two, three, four, or five amino acid residues Attorney Docket No.48881-0047WO1 / 05958 directly preceding SEQ ID NO:19, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:19, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:19 include, without limitation, those set forth in Table 9. Table 9. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:19. Sequence SEQ ID NO: In another embodiment, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:25 (or a variant of SEQ ID NO:25 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:26 (or a variant of SEQ ID NO:26 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:27 (or a variant of SEQ ID NO:27 with Attorney Docket No.48881-0047WO1 / 05958 one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 6. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:25 (or a variant of SEQ ID NO:25 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:26 (or a variant of SEQ ID NO:26 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:27 (or a variant of SEQ ID NO:27 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:28 (or a variant of SEQ ID NO:28 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:29 (or a variant of SEQ ID NO:29 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:30 (or a variant of SEQ ID NO:30 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:31 (or a variant of SEQ ID NO:31 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 6 can be designed to include framework regions as set forth in Example 6 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 6 and the framework regions set forth in Attorney Docket No.48881-0047WO1 / 05958 Example 6 except that framework region 1 having the amino acid set forth in SEQ ID NO:28 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:20. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 6, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:32. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:32. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:32. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:32, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:25, 26, and 27. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in Attorney Docket No.48881-0047WO1 / 05958 SEQ ID NO:32, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:25, 26, and 27. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:32 or the amino acid set forth in SEQ ID NO:32 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:32 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:25, 26, and 27. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:25, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:26, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:27. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:25” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:25, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:25, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:25, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists Attorney Docket No.48881-0047WO1 / 05958 essentially of the amino acid sequence set forth in SEQ ID NO:25 include, without limitation, those set forth in Table 10. Table 10. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:25. Sequence SEQ ID NO: GFTFDDYA 340 As used herein, a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:26” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:26, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:26, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:26, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:26 include, without limitation, those set forth in Table 11. Table 11. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:26. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 INPNSGGT 350 INPSGGST 351 s use ere n, a t at cons sts essent a y o t e amno ac sequence set forth in SEQ ID NO:27” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:27, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:27, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:27, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:27 include, without limitation, those set forth in Table 12. Table 12. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:27. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 ARIGLRSSGSRESHPI 365 AKDGLRSSGSRESHPI 366 , . ., , , antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:33 (or a variant of SEQ ID NO:33 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:34 (or a variant of SEQ ID NO:34 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:35 (or a variant of SEQ ID NO:35 with one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 7. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:33 (or a variant of SEQ ID NO:33 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:34 (or a variant of SEQ ID NO:34 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:35 (or a variant of SEQ ID NO:35 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:36 (or a variant of SEQ ID NO:36 with one, two, three, four, five, six, seven, eight, nine, ten, or Attorney Docket No.48881-0047WO1 / 05958 more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:37 (or a variant of SEQ ID NO:37 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:38 (or a variant of SEQ ID NO:38 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:39 (or a variant of SEQ ID NO:39 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 7 can be designed to include framework regions as set forth in Example 7 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 7 and the framework regions set forth in Example 7 except that framework region 1 having the amino acid set forth in SEQ ID NO:36 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:12, a framework region 1 having the amino acid set forth in SEQ ID NO:20, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 7, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:40. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:40. In some cases, a Attorney Docket No.48881-0047WO1 / 05958 binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:40. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:40, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:33, 34, and 35. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:40, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs: 33, 34, and 35. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:40 or the amino acid set forth in SEQ ID NO:40 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:40 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs: 33, 34, and 35. Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:33, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:34, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:35. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:33” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:33, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:33, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:33, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:33 include, without limitation, those set forth in Table 13. Table 13. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:33 Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 GGSISSGYY238 et forth in SEQ ID NO:34” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:34, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:34, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:34, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:34 include, without limitation, those set forth in Table 14. Table 14. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:34. Sequence SEQ ID NO: As used herein, a “CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:35” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:35, that has zero, one, two, three, four, or five amino acid residues Attorney Docket No.48881-0047WO1 / 05958 directly preceding SEQ ID NO:35, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:35, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:35 include, without limitation, those set forth in Table 15. Table 15. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:35. Sequence SEQ ID NO: In another embodiment, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:41 (or a variant of SEQ ID NO:41 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:42 (or a variant of SEQ ID NO:42 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:43 (or a variant of SEQ ID NO:43 with Attorney Docket No.48881-0047WO1 / 05958 one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 8. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:41 (or a variant of SEQ ID NO:41 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:42 (or a variant of SEQ ID NO:42 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:43 (or a variant of SEQ ID NO:43 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:44 (or a variant of SEQ ID NO:44 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:45 (or a variant of SEQ ID NO:45 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:46 (or a variant of SEQ ID NO:46 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:47 (or a variant of SEQ ID NO:47 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 8 can be designed to include framework regions as set forth in Example 8 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 8 and the framework regions set forth in Attorney Docket No.48881-0047WO1 / 05958 Example 8 except that framework region 1 having the amino acid set forth in SEQ ID NO:44 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:20, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 8, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:48. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:48. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:48. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:48, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:41, 42, and 43. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in Attorney Docket No.48881-0047WO1 / 05958 SEQ ID NO:48, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:41, 42, and 43. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:48 or the amino acid set forth in SEQ ID NO:48 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:48 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:41, 42, and 43. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:41, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:42, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:43. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:41” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:41, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:41, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:41, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists Attorney Docket No.48881-0047WO1 / 05958 essentially of the amino acid sequence set forth in SEQ ID NO:41 include, without limitation, those set forth in Table 16. Table 16. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:41. Sequence SEQ ID NO: 379 As used herein, a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:42” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:42, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:42, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:42, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:42 include, without limitation, those set forth in Table 17. Table 17. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:42. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 INPNSGGT 389 INPSGGST 390 s use ere n, a t at cons sts essent a y o t e amno ac sequence set forth in SEQ ID NO:43” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:43, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:43, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:43, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:43 include, without limitation, those set forth in Table 18. Table 18. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:43. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 ARIGAPREMATI 404 AKDGAPREMATI 405 , . ., , , antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:49 (or a variant of SEQ ID NO:49 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:50 (or a variant of SEQ ID NO:50 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:51 (or a variant of SEQ ID NO:51 with one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 9. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:49 (or a variant of SEQ ID NO:49 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:50 (or a variant of SEQ ID NO:50 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:51 (or a variant of SEQ ID NO:51 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:52 (or a variant of SEQ ID NO:52 with one, two, three, four, five, six, seven, eight, nine, ten, or Attorney Docket No.48881-0047WO1 / 05958 more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:53 (or a variant of SEQ ID NO:53 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:54 (or a variant of SEQ ID NO:54 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:55 (or a variant of SEQ ID NO:55 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 9 can be designed to include framework regions as set forth in Example 9 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 9 and the framework regions set forth in Example 9 except that framework region 1 having the amino acid set forth in SEQ ID NO:52 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 9, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:56. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:56. In some cases, a Attorney Docket No.48881-0047WO1 / 05958 binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:56. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:56, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:49, 50, and 51. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:56, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:49, 50, and 51. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:56 or the amino acid set forth in SEQ ID NO:56 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:56 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:49, 50, and 51. Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:49, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:50, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:51. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:49” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:49, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:49, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:49, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:49 include, without limitation, those set forth in Table 19. Table 19. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:49. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 GGSISSGYY 418 et forth in SEQ ID NO:50” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:50, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:50, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:50, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:50 include, without limitation, those set forth in Table 20. Table 20. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:50. Sequence SEQ ID NO: As used herein, a “CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:51” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:51, that has zero, one, two, three, four, or five amino acid residues Attorney Docket No.48881-0047WO1 / 05958 directly preceding SEQ ID NO:51, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:51, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:51 include, without limitation, those set forth in Table 21. Table 21. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:51. Sequence SEQ ID NO: In another embodiment, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:57 (or a variant of SEQ ID NO:57 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:58 (or a variant of SEQ ID NO:58 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:59 (or a variant of SEQ ID NO:59 with Attorney Docket No.48881-0047WO1 / 05958 one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 10. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:57 (or a variant of SEQ ID NO:57 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:58 (or a variant of SEQ ID NO:58 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:59 (or a variant of SEQ ID NO:59 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:60 (or a variant of SEQ ID NO:60 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:61 (or a variant of SEQ ID NO:61 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:62 (or a variant of SEQ ID NO:62 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:63 (or a variant of SEQ ID NO:63 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 10 can be designed to include framework regions as set forth in Example 10 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 10 and the framework regions Attorney Docket No.48881-0047WO1 / 05958 set forth in Example 10 except that framework region 1 having the amino acid set forth in SEQ ID NO:60 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:20. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 10, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:64. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:64. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:64. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:64, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:57, 58, and 59. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in Attorney Docket No.48881-0047WO1 / 05958 SEQ ID NO:64, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:57, 58, and 59. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:64 or the amino acid set forth in SEQ ID NO:64 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:64 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:57, 58, and 59. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:57, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:58, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:59. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:57” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:57, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:57, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:57, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists Attorney Docket No.48881-0047WO1 / 05958 essentially of the amino acid sequence set forth in SEQ ID NO:57 include, without limitation, those set forth in Table 22. Table 22. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:57. Sequence SEQ ID NO: GFTFDDYA 439 As used herein, a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:58” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:58, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:58, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:58, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:58 include, without limitation, those set forth in Table 23. Table 23. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:58. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 INPNSGGT 449 INPSGGST 450 s use ere n, a t at cons sts essent a y o t e amno ac sequence set forth in SEQ ID NO:59” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:59, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:59, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:59, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:59 include, without limitation, those set forth in Table 24. Table 24. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:59. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 ARIGAFYTWNL 464 AKDGAFYTWNL 465 , . ., , , antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:65 (or a variant of SEQ ID NO:65 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:66 (or a variant of SEQ ID NO:66 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:67 (or a variant of SEQ ID NO:67 with one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 11. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:65 (or a variant of SEQ ID NO:65 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:66 (or a variant of SEQ ID NO:66 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:67 (or a variant of SEQ ID NO:67 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:68 (or a variant of SEQ ID NO:68 with one, two, three, four, five, six, seven, eight, nine, ten, or Attorney Docket No.48881-0047WO1 / 05958 more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:69 (or a variant of SEQ ID NO:69 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:70 (or a variant of SEQ ID NO:70 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:71 (or a variant of SEQ ID NO:71 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 11 can be designed to include framework regions as set forth in Example 11 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 11 and the framework regions set forth in Example 11 except that framework region 1 having the amino acid set forth in SEQ ID NO:68 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:20. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 11, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:72. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:72. In some cases, a Attorney Docket No.48881-0047WO1 / 05958 binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:72. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:72, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:65, 66, and 67. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:72, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:65, 66, and 67. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:72 or the amino acid set forth in SEQ ID NO:72 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:72 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:65, 66, and 67. Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:65, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:66, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:67. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:65” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:65, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:65, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:65, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:65 include, without limitation, those set forth in Table 25. Table 25. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:65. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 GGSISSGYY478 et forth in SEQ ID NO:66” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:66, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:66, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:66, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:66 include, without limitation, those set forth in Table 26. Table 26. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:66. Sequence SEQ ID NO: As used herein, a “CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:67” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:67, that has zero, one, two, three, four, or five amino acid residues Attorney Docket No.48881-0047WO1 / 05958 directly preceding SEQ ID NO:67, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:67, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:67 include, without limitation, those set forth in Table 27. Table 27. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:67. Sequence SEQ ID NO: In another embodiment, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:73 (or a variant of SEQ ID NO:73 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:74 (or a variant of SEQ ID NO:74 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:75 (or a variant of SEQ ID NO:75 with Attorney Docket No.48881-0047WO1 / 05958 one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 12. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:73 (or a variant of SEQ ID NO:73 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:74 (or a variant of SEQ ID NO:74 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:75 (or a variant of SEQ ID NO:75 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:76 (or a variant of SEQ ID NO:76 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:77 (or a variant of SEQ ID NO:77 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:78 (or a variant of SEQ ID NO:78 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:79 (or a variant of SEQ ID NO:79 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 12 can be designed to include framework regions as set forth in Example 12 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 12 and the framework regions Attorney Docket No.48881-0047WO1 / 05958 set forth in Example 12 except that framework region 1 having the amino acid set forth in SEQ ID NO:76 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:20, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 12, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:80. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:80. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:80. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:80, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:73, 74, and 75. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in Attorney Docket No.48881-0047WO1 / 05958 SEQ ID NO:80, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:73, 74, and 75. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:80 or the amino acid set forth in SEQ ID NO:80 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:80 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:73, 74, and 75. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:73, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:74, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:75. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:73” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:73, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:73, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:73, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists Attorney Docket No.48881-0047WO1 / 05958 essentially of the amino acid sequence set forth in SEQ ID NO:73 include, without limitation, those set forth in Table 28. Table 28. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:73. Sequence SEQ ID NO: 499 As used herein, a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:74” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:74, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:74, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:74, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:74 include, without limitation, those set forth in Table 29. Table 29. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:74. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 INPNSGGT 509 INPSGGST 510 s use ere n, a t at cons sts essent a y o t e amno ac sequence set forth in SEQ ID NO:75” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:75, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:75, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:75, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:75 include, without limitation, those set forth in Table 30. Table 30. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:75. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 ARIDGYYGMMV 524 AKDGDGYYGMDV 525 , . ., , , antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:81 (or a variant of SEQ ID NO:81 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:82 (or a variant of SEQ ID NO:82 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:83 (or a variant of SEQ ID NO:83 with one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 13. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:81 (or a variant of SEQ ID NO:81 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:82 (or a variant of SEQ ID NO:82 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:83 (or a variant of SEQ ID NO:83 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:84 (or a variant of SEQ ID NO:84 with one, two, three, four, five, six, seven, eight, nine, ten, or Attorney Docket No.48881-0047WO1 / 05958 more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:85 (or a variant of SEQ ID NO:85 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:86 (or a variant of SEQ ID NO:86 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:87 (or a variant of SEQ ID NO:87 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 13 can be designed to include framework regions as set forth in Example 13 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 13 and the framework regions set forth in Example 13 except that framework region 1 having the amino acid set forth in SEQ ID NO:84 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 13, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:88. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:88. In some cases, a Attorney Docket No.48881-0047WO1 / 05958 binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:88. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:88, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:81, 82, and 83. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:88, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:81, 82, and 83. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:88 or the amino acid set forth in SEQ ID NO:88 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:88 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:81, 82, and 83. Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:81, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:82, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:83. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:81” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:81, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:81, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:81, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:81 include, without limitation, those set forth in Table 31. Table 31. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:81. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 GGSISSGYY 538 et forth in SEQ ID NO:82” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:82, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:82, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:82, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:82 include, without limitation, those set forth in Table 32. Table 32. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:82. Sequence SEQ ID NO: As used herein, a “CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:83” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:83, that has zero, one, two, three, four, or five amino acid residues Attorney Docket No.48881-0047WO1 / 05958 directly preceding SEQ ID NO:83, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:83, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:83 include, without limitation, those set forth in Table 33. Table 33. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:83. Sequence SEQ ID NO: In another embodiment, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:89 (or a variant of SEQ ID NO:89 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:90 (or a variant of SEQ ID NO:90 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:91 (or a variant of SEQ ID NO:91 with Attorney Docket No.48881-0047WO1 / 05958 one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 14. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:89 (or a variant of SEQ ID NO:89 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:90 (or a variant of SEQ ID NO:90 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:91 (or a variant of SEQ ID NO:91 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:92 (or a variant of SEQ ID NO:92 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:93 (or a variant of SEQ ID NO:93 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:94 (or a variant of SEQ ID NO:94 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:95 (or a variant of SEQ ID NO:95 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 14 can be designed to include framework regions as set forth in Example 14 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 14 and the framework regions Attorney Docket No.48881-0047WO1 / 05958 set forth in Example 14 except that framework region 1 having the amino acid set forth in SEQ ID NO:92 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:20. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 14, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:96. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:96. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:96. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:96, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:89, 90, and 91. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in Attorney Docket No.48881-0047WO1 / 05958 SEQ ID NO:96, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:89, 90, and 91. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:96 or the amino acid set forth in SEQ ID NO:96 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:96 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:89, 90, and 91. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:89, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:90, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:91. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:89” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:89, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:89, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:89, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists Attorney Docket No.48881-0047WO1 / 05958 essentially of the amino acid sequence set forth in SEQ ID NO:89 include, without limitation, those set forth in Table 34. Table 34. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:89. Sequence SEQ ID NO: GFTFDDYA 559 As used herein, a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:90” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:90, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:90, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:90, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:90 include, without limitation, those set forth in Table 35. Table 35. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:90. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 INPNSGGT 569 INPSGGST 570 s use ere n, a t at cons sts essent a y o t e amno ac sequence set forth in SEQ ID NO:91” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:91, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:91, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:91, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:91 include, without limitation, those set forth in Table 36. Table 36. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:91. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 ARIGRRVYDSSGYYVMV 584 AKDGRRVYDSSGYYV 585 , . ., , , antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:97 (or a variant of SEQ ID NO:97 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:98 (or a variant of SEQ ID NO:98 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:99 (or a variant of SEQ ID NO:99 with one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 15. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:97 (or a variant of SEQ ID NO:97 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:98 (or a variant of SEQ ID NO:98 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:99 (or a variant of SEQ ID NO:99 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:100 (or a variant of SEQ ID NO:100 with one, two, three, four, five, six, seven, eight, nine, ten, or Attorney Docket No.48881-0047WO1 / 05958 more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:101 (or a variant of SEQ ID NO:101 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:102 (or a variant of SEQ ID NO:102 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:103 (or a variant of SEQ ID NO:103 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 15 can be designed to include framework regions as set forth in Example 15 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 15 and the framework regions set forth in Example 15 except that framework region 1 having the amino acid set forth in SEQ ID NO:100 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:28. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 15, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:104. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:104. In some cases, Attorney Docket No.48881-0047WO1 / 05958 a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:104. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:104, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:97, 98, and 99. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:104, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:97, 98, and 99. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:104 or the amino acid set forth in SEQ ID NO:104 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:104 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:97, 98, and 99. Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:97, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:98, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:99. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:97” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:97, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:97, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:97, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:97 include, without limitation, those set forth in Table 37. Table 37. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:97. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 GGSISSGYY 598 et forth in SEQ ID NO:98” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:98, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:98, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:98, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:98 include, without limitation, those set forth in Table 38. Table 38. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:98. Sequence SEQ ID NO: As used herein, a “CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:99” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:99, that has zero, one, two, three, four, or five amino acid residues Attorney Docket No.48881-0047WO1 / 05958 directly preceding SEQ ID NO:99, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:99, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:99 include, without limitation, those set forth in Table 39. Table 39. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:99. Sequence SEQ ID NO: In another embodiment, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:105 (or a variant of SEQ ID NO:105 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:106 (or a variant of SEQ ID NO:106 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:107 (or a variant of SEQ ID Attorney Docket No.48881-0047WO1 / 05958 NO:107 with one or two amino acid modifications). An example of such a binder having these CDRs and the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) includes, without limitation, the VH domain set forth in Example 16. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and having a heavy chain variable domain having a CDR1 having the amino acid sequence set forth in SEQ ID NO:105 (or a variant of SEQ ID NO:105 with one or two amino acid modifications), a CDR2 having the amino acid sequence set forth in SEQ ID NO:106 (or a variant of SEQ ID NO:106 with one or two amino acid modifications), and a CDR3 having the amino acid sequence set forth in SEQ ID NO:107 (or a variant of SEQ ID NO:107 with one or two amino acid modifications) can include any appropriate framework regions. For example, such a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) can include a heavy chain variable domain that includes a framework region 1 having the amino acid sequence set forth in SEQ ID NO:108 (or a variant of SEQ ID NO:108 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 2 having the amino acid sequence set forth in SEQ ID NO:109 (or a variant of SEQ ID NO:109 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), a framework region 3 having the amino acid sequence set forth in SEQ ID NO:110 (or a variant of SEQ ID NO:110 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications), and a framework region 4 having the amino acid sequence set forth in SEQ ID NO:111 (or a variant of SEQ ID NO:111 with one, two, three, four, five, six, seven, eight, nine, ten, or more amino acid modifications). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) having any of the CDRs set forth in Example 16 can be designed to include framework regions as set forth in Example 16 or can be designed to include one or more framework regions from another antibody or antibody fragment. For example, an antibody domain (e.g., a VH domain) can be designed to include the three CDRs set forth in Example 16 and the framework regions Attorney Docket No.48881-0047WO1 / 05958 set forth in Example 16 except that framework region 1 having the amino acid set forth in SEQ ID NO:108 is replaced with a framework region 1 having the amino acid set forth in SEQ ID NO:4, a framework region 1 having the amino acid set forth in SEQ ID NO:12, or a framework region 1 having the amino acid set forth in SEQ ID NO:20. In another example, an Fab or scFv can be designed to include (a) the three CDRs set forth in Example 16, (b) the framework regions set forth in Examples 3-16, and (c) a light chain variable domain. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:112. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence set forth in SEQ ID NO:112. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include (a) a heavy chain variable domain that includes an amino acid sequence having 100 percent identity to the amino acid sequence set forth in SEQ ID NO:112. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain that includes an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:112, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:105, 106, and 107. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein can include a heavy chain variable domain that includes an amino acid sequence having at least 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99 percent identity to the amino acid sequence Attorney Docket No.48881-0047WO1 / 05958 set forth in SEQ ID NO:112, provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:105, 106, and 107. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:112 or the amino acid set forth in SEQ ID NO:112 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions). For example, antibody domain (e.g., a VH domain) provided herein can have the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and can include a heavy chain variable domain having the amino acid sequence set forth in SEQ ID NO:112 with one, two, three, four, five, six, seven, eight, nine, or 10 amino acid modifications (e.g., amino acid substitutions, amino acid deletions, and / or amino acid additions), provided that the heavy chain variable domain includes the amino acid sequences set forth in SEQ ID NOs:105, 106, and 107. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can include a heavy chain variable domain comprising (i) a CDR1 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:105, (ii) a CDR2 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:106, and (iii) a CDR3 that comprises, consists essentially of, or consists of the amino acid sequence set forth in SEQ ID NO:107. As used herein, a “CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:105” is a CDR1 that has zero, one, or two amino acid substitutions within SEQ ID NO:105, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:105, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:105, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a Attorney Docket No.48881-0047WO1 / 05958 follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR1 that consists essentially of the amino acid sequence set forth in SEQ ID NO:105 include, without limitation, those set forth in Table 40. Table 40. Exemplary CDR1s that consist essentially of the amino acid sequence set forth in SEQ ID NO:105. Sequence SEQ ID NO: GFTFDDYA 619 As used herein, a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:106” is a CDR2 that has zero, one, or two amino acid substitutions within SEQ ID NO:106, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:106, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:106, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR2 that consists essentially of the amino acid sequence set forth in SEQ ID NO:106 include, without limitation, those set forth in Table 41. Table 41. Exemplary CDR2s that consist essentially of the amino acid sequence set forth in SEQ ID NO:106. Attorney Docket No.48881-0047WO1 / 05958 Sequence SEQ ID NO: INPNSGGT 629 s used ere n, a C 3 t at cons sts essent a y o t e amno acd sequence set forth in SEQ ID NO:107” is a CDR3 that has zero, one, or two amino acid substitutions within SEQ ID NO:107, that has zero, one, two, three, four, or five amino acid residues directly preceding SEQ ID NO:107, and / or that has zero, one, two, three, four, or five amino acid residues directly following SEQ ID NO:107, provided that the binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) maintains its basic ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of a CDR3 that consists essentially of the amino acid sequence set forth in SEQ ID NO:107 include, without limitation, those set forth in Table 42. Table 42. Exemplary CDR3s that consist essentially of the amino acid sequence set forth in SEQ ID NO:107. Sequence SEQ ID NO: Attorney Docket No.48881-0047WO1 / 05958 ARRGLFSGVPL 643 ARILFSGVPLL 644 . ., variable domain and a light chain variable domain, the two regions can be directly connected or can be connected using any appropriate linker sequence. For example, a heavy chain variable domain can be connected to a light chain variable domain via a linker sequence. Examples of linker sequences that can be used to connect a heavy chain variable domain and a light chain variable domain to create a scFv include, without limitation, those linkers set forth in Example 20. As indicated herein, the amino acid sequences described herein can include amino acid modifications (e.g., the articulated number of amino acid modifications). Such amino acid modifications can include, without limitation, amino acid substitutions, amino acid deletions, amino acid additions, and combinations. In some cases, an amino acid modification can be made to improve the binding and / or contact with an antigen and / or to improve a functional activity of a binder (e.g., an antibody, antigen binding fragment, antibody domain, a CAR, a cell engager, and / or an ADC) provided herein. In some cases, an amino acid substitution within an articulated sequence identifier can be a conservative amino acid substitution. For example, conservative amino acid substitutions can be made by substituting one amino acid residue for another amino acid residue having a similar side chain. Families of amino acid residues having similar side chains can include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), non-polar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta- Attorney Docket No.48881-0047WO1 / 05958 branched side chains (e.g., threonine, valine, isoleucine), and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine). In some cases, an amino acid substitution within an articulated sequence identifier can be a non-conservative amino acid substitution. Non-conservative amino acid substitutions can be made by substituting one amino acid residue for another amino acid residue having a dissimilar side chain. Examples of non-conservative substitutions include, without limitation, substituting (a) a hydrophilic residue (e.g., serine or threonine) for a hydrophobic residue (e.g., leucine, isoleucine, phenylalanine, valine, or alanine); (b) a cysteine or proline for any other residue; (c) a residue having a basic side chain (e.g., lysine, arginine, or histidine) for a residue having an acidic side chain (e.g., aspartic acid or glutamic acid); and (d) a residue having a bulky side chain (e.g., phenylalanine) for glycine or other residue having a small side chain. The percent sequence identity between a particular amino acid or nucleic acid sequence and an amino acid or nucleic acid sequence referenced by a particular sequence identification number is determined as follows. First, an amino acid or nucleic acid sequence is compared to the sequence set forth in a particular sequence identification number using the BLAST 2 Sequences (Bl2seq) program from the stand-alone version of BLASTZ containing BLASTN version 2.0.14 and BLASTP version 2.0.14. This stand- alone version of BLASTZ can be obtained from Fish & Richardson’s web site (e.g., www.fr.com / blast / ) or the U.S. government’s National Center for Biotechnology Information web site (www.ncbi.nlm.nih.gov). Instructions explaining how to use the Bl2seq program can be found in the readme file accompanying BLASTZ. Bl2seq performs a comparison between two sequences using either the BLASTN or BLASTP algorithm. BLASTN is used to compare nucleic acid sequences, while BLASTP is used to compare amino acid sequences. To compare two nucleic acid sequences, the options are set as follows: -i is set to a file containing the first nucleic acid sequence to be compared (e.g., C:\seq1.txt); -j is set to a file containing the second nucleic acid sequence to be compared (e.g., C:\seq2.txt); -p is set to blastn; -o is set to any desired file name (e.g., C:\output.txt); -q is set to -1; -r is set to 2; and all other options are left at their default setting. For example, the following command can be used to generate an output Attorney Docket No.48881-0047WO1 / 05958 file containing a comparison between two sequences: C:\Bl2seq -i c:\seq1.txt -j c:\seq2.txt -p blastn -o c:\output.txt -q -1 -r 2. To compare two amino acid sequences, the options of Bl2seq are set as follows: -i is set to a file containing the first amino acid sequence to be compared (e.g., C:\seq1.txt); -j is set to a file containing the second amino acid sequence to be compared (e.g., C:\seq2.txt); -p is set to blastp; -o is set to any desired file name (e.g., C:\output.txt); and all other options are left at their default setting. For example, the following command can be used to generate an output file containing a comparison between two amino acid sequences: C:\Bl2seq -i c:\seq1.txt -j c:\seq2.txt -p blastp -o c:\output.txt. If the two compared sequences share homology, then the designated output file will present those regions of homology as aligned sequences. If the two compared sequences do not share homology, then the designated output file will not present aligned sequences. Once aligned, the number of matches is determined by counting the number of positions where an identical nucleotide or amino acid residue is presented in both sequences. A matched position refers to a position in which an identical nucleotide or amino acid residue occurs at the same position in aligned sequences. The percent sequence identity is determined by dividing the number of matches by the length of the sequence set forth in the identified sequence (e.g., SEQ ID NO:8, SEQ ID NO:16, or SEQ ID NO:24), followed by multiplying the resulting value by 100. For example, an amino acid sequence that has 100 matches when aligned with the sequence set forth in SEQ ID NO:8 is 80 percent identical to the sequence set forth in SEQ ID NO:8 (i.e., 100 ÷ 125 x 100 = 80.0). It is noted that the percent sequence identity value is rounded to the nearest tenth. For example, 78.11, 78.12, 78.13, and 78.14 is rounded down to 78.1, while 78.15, 78.16, 78.17, 78.18, and 78.19 is rounded up to 78.2. It also is noted that the length value will always be an integer. Methods for generating an amino acid sequence variant (e.g., an amino acid sequence that includes one or more modifications with respect to an articulated sequence identifier) can include site-specific mutagenesis or random mutagenesis (e.g., by PCR) of a nucleic acid encoding the antibody or fragment thereof. See, for example, Zoller, Curr. Opin. Biotechnol.3: 348-354 (1992). Both naturally occurring and non-naturally Attorney Docket No.48881-0047WO1 / 05958 occurring amino acids (e.g., artificially-derivatized amino acids) can be used to generate an amino acid sequence variant provided herein. A representative number of binders (e.g., antibodies, antigen binding fragments, and / or antibody domains) having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) are further described in Table 43. Table 43. Representative number of binders. Clone # (Antibody type) SEQ ID NOs of Heavy SEQ ID NOs of Heavy SEQ ID NO of Chain Variable Domain Chain Variable Domain Heavy Chain The binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, cell engagers, and / or ADCs) provided herein can be produced using any appropriate method. For example, the binders (e.g., antibodies, antigen binding fragments, antibody domains, CARs, and / or cell engagers) provided herein can be produced in recombinant host cells. For example, a nucleic acid encoding a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein can be constructed, introduced into an expression vector, and expressed in suitable host cells. Example 17 is a sequence listing of nucleic acid sequences Attorney Docket No.48881-0047WO1 / 05958 encoding exemplary binders (e.g., antibodies, antigen binding fragments, and / or antibody domains) described herein. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein can be recombinantly produced in prokaryotic hosts such as E. coli, Bacillus brevis, Bacillus subtilis, Bacillus megaterium, Lactobacillus zeae / casei, or Lactobacillus paracasei. A binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein also can be recombinantly produced in eukaryotic hosts such as yeast (e.g., Pichia pastoris, Saccharomyces cerevisiae, Hansenula polymorpha, Schizosaccharomyces pombe, Schwanniomyces occidentalis, Kluyveromyces lactis, or Yarrowia lipolytica), filamentous fungi of the genera Trichoderma (e.g., T. reesei) and Aspergillus (e.g., A. niger and A. oryzae), protozoa such as Leishmania tarentolae, insect cells, or mammalian cells (e.g., mammalian cell lines such as Chinese hamster ovary (CHO) cells, Per.C6 cells, mouse myeloma NS0 cells, baby hamster kidney (BHK) cells, or human embryonic kidney cell line HEK293). See, for example, the Frenzel et al. reference (Front Immunol., 4:217 (2013)). In some cases, an antigen binding fragment or antibody domain provided herein can be produced by proteolytic digestion of an intact antibody. For example, an antigen binding fragment can be obtained by treating an antibody with an enzyme such as papain or pepsin. Papain digestion of whole antibodies can be used to produce F(ab)2or Fab fragments, while pepsin digestion of whole antibodies can be used to produce F(ab’)2or Fab’ fragments. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein can be substantially pure. The term “substantially pure” as used herein with reference to a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) refers to the binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) as being substantially free of other polypeptides, lipids, carbohydrates, and nucleic acid with which it is naturally associated. Thus, a substantially pure binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein is any binder (e.g., an antibody, antigen Attorney Docket No.48881-0047WO1 / 05958 binding fragment, antibody domain, CAR, cell engager, and / or ADC) that is removed from its natural environment and is at least 60 percent pure. A substantially pure binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein can be at least about 65, 70, 75, 80, 85, 90, 95, or 99 percent pure. This document also provides bispecific binders (e.g., bispecific antibodies, bispecific antigen binding fragments, and / or bispecific antibody domains) that bind to two different epitopes with at least one being an epitope of a follistatin polypeptide (e.g., a human follistatin polypeptide). In some cases, a bispecific binder provided herein can be designed to bind to two different epitopes of the same follistatin polypeptide (e.g., a human follistatin polypeptide). In some cases, a bispecific binder provided herein can bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and to an epitope on a different polypeptide (e.g., a CD3 polypeptide). Bispecific binders can be produced by chemically conjugating two different binders (e.g., antibodies, antigen binding fragments, and / or antibody domains) together. Bispecific binders also can be produced by fusing two antibody-producing cells, e.g., hybridomas, to make a hybrid cell line that produces two different heavy and two different light chains within the same cell, which can result in, for example, bispecific IgG molecules. See, Brinkmann and Kontermann, MAbs., 9(2):182-212 (2017). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein can be fused or conjugated (e.g., covalently or non-covalently attached) to another polypeptide or other moiety to provide a fusion protein or conjugate. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein can be conjugated (e.g., covalently or non-covalently attached) to a polymer (e.g., polyethylene glycol (PEG), polyethylenimine (PEI) modified with PEG (PEI-PEG), and / or polyglutamic acid (PGA) (N-(2-Hydroxypropyl) methacrylamide (HPMA) copolymers), hyaluronic acid, a fluorescent substance, a luminescent substance, a hapten, an enzyme, a metal chelate, a drug, a radioisotope, and / or a cytotoxic agent. Any appropriate method can be used to conjugate (e.g., covalently or non-covalently attach) another polypeptide or other moiety to a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or Attorney Docket No.48881-0047WO1 / 05958 cell engager) provided herein. For example, another polypeptide or other moiety can be conjugated to a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein using the methods described in U.S. Patent No.8,021,661. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein can be modified with a moiety that improves its stabilization and / or retention in circulation, for example, in blood, serum, or other tissues by, for example, at least 1.5-, 2-, 5-, 10-, or 50-fold. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein can be attached (e.g., covalently or non-covalently attached) to a polymer such as a substantially non-antigenic polymer. Examples of substantially non-antigenic polymers that can be used as described herein include, without limitation, polyalkylene oxides and polyethylene oxides. In some cases, a polymer used herein can have any appropriate molecule weight. For example, a polymer having an average molecular weight from about 200 Daltons to about 35,000 Daltons (e.g., from about 1,000 to about 15,000 Daltons or from about 2,000 to about 12,500 Daltons) can be used. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein can be attached (e.g., covalently or non-covalently) to a water soluble polymer. Examples of water soluble polymers that can be used as described herein include, without limitation, hydrophilic polyvinyl polymers, polyvinylalcohol, polyvinylpyrrolidone, polyalkylene oxide homopolymers, polyethylene glycol (PEG), polypropylene glycols, polyoxyethylenated polyols, and copolymers thereof and / or block copolymers thereof provided that the water solubility of the copolymer or block copolymers is maintained. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein can be attached (e.g., covalently or non-covalently attached) to one or more polyoxyalkylenes (e.g., polyoxyethylene, polyoxypropylene, or block copolymers of polyoxyethylene and polyoxypropylene), polymethacrylates, carbomers, branched or unbranched polysaccharides, or combinations thereof. For example, a binder (e.g., an antibody, Attorney Docket No.48881-0047WO1 / 05958 antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein can be covalently attached to polyoxyethylene. This document also provides ADCs. The term “ADC” as used herein refers to a conjugate that includes (a) an antigen binding domain and (b) at least one drug covalently linked directly or indirectly to that antigen binding domain. In some cases, an ADC described herein can include (a) an antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) and (b) at least one drug covalently linked directly or indirectly to that antigen binding domain. Any appropriate binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein and having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can be used as an antigen binding domain to make an ADC described herein. For example, any of the binders set forth in Table 43 can be used to make an ADC having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide). Examples of drugs that can be used to make an ADC described herein include, without limitation, auristatins (e.g., monomethyl auristatin E (MMAE)), mertansine (DM-1), and pyrrolobenzodiazepine (PBD) dimers. Any appropriate ADC linker can be used to covalently attach one or more drugs to an antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) to form an ADC provided herein. For example, cleavable or non-cleavable ADC linkers can be used to covalently attach one or more drugs to an antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) to form an ADC provided herein. Examples of ADC linkers can be used to covalently attach one or more drugs to an antigen binding domain having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) to form an ADC provided herein include, without limitation, ADC disulfide linkers, ADC hydrazone linkers, ADC peptide linkers, ADC thioether linkers, and ADC PEG-containing linkers. This document also provides nucleic acid molecules (e.g., isolated nucleic acid molecules) having a nucleic acid sequence encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein. For example, an isolated nucleic acid molecule provided herein can Attorney Docket No.48881-0047WO1 / 05958 include a nucleic acid sequence encoding a VH domain such as a VH domain as set forth in any one of Examples 3-16. In another example, an isolated nucleic acid molecule provided herein can include a nucleic acid sequence encoding a CAR or cell engager (e.g., a BiTE, BiKE, or TriKE) described herein. A nucleic acid provided herein (e.g., an isolated nucleic acid molecule) can be single stranded or double stranded nucleic acid of any appropriate type (e.g., DNA, RNA, or DNA / RNA hybrids). This document also provides vectors (e.g., plasmid vectors or viral vectors) containing one or more nucleic acids provided herein. An example of a plasmid vector that can be designed to include one or more nucleic acids having a nucleic acid sequence encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein includes, without limitation, phagemids. Examples of viral vectors that can be designed to include one or more nucleic acids having a nucleic acid sequence encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein include, without limitation, retroviral vectors, parvovirus-based vectors (e.g., adenoviral-based vectors and adeno-associated virus (AAV)-based vectors), lentiviral vectors (e.g., herpes simplex (HSV)-based vectors), poxviral vectors (e.g., vaccinia virus-based vectors and fowlpox virus-based vectors), and hybrid or chimeric viral vectors. For example, a viral vector having an adenoviral backbone with lentiviral components such as those described elsewhere (Zheng et al., Nat. Biotech., 18(2): 176-80 (2000); WO 98 / 22143; WO 98 / 46778; and WO 00 / 17376) or viral vectors having an adenoviral backbone with AAV components such as those described elsewhere (Fisher et al., Hum. Gene Ther., 7:2079-2087 (1996)) can be designed to include one or more nucleic acids having a nucleic acid sequence encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein. In some cases, a vector (e.g., a plasmid vector or a viral vector) provided herein can include a nucleic acid sequence encoding scFv or antibody domain (e.g., a VH domain) provided herein. In some cases, a vector (e.g., a plasmid vector or a viral vector) provided herein can include a nucleic acid sequence encoding CAR provided herein. In Attorney Docket No.48881-0047WO1 / 05958 some cases, a vector (e.g., a plasmid vector or a viral vector) provided herein can include a nucleic acid sequence encoding cell engager provided herein. A vector provided herein (e.g., a plasmid vector or viral vector provided herein) can include any appropriate promoter and other regulatory sequence (e.g., transcription and translation initiation and termination codons) operably linked the nucleic acid sequence encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein. In some cases, a promoter used to drive expression can be a constitutive promotor or a regulatable promotor. Examples of regulatable promoters that can be used as described herein include, without limitation, inducible promotors, repressible promotors, and tissue-specific promoters. Examples of viral promotors that can be used as described herein include, without limitation, adenoviral promotors, vaccinia virus promotors, CMV promotors (e.g., immediate early CMV promotors), and AAV promoters. Any appropriate method can be used to make a nucleic acid molecule (or vector such as a plasmid vector or viral vector) having a nucleic acid sequence encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein. For example, molecule cloning techniques can be used to make a nucleic acid molecule (or vector such as a plasmid vector or viral vector) having a nucleic acid sequence encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein as described elsewhere (see, e.g., Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd edition, Cold Spring Harbor Laboratory, NY (1989); and Ausubel et al., Current Protocols in Molecular Biology, Green Publishing Associates and John Wiley & Sons, New York, N.Y. (1994)). This document also provides host cells that include a nucleic acid provided herein (e.g., a nucleic acid having a nucleic acid sequence encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein). Host cells that can be designed to include one or more nucleic acids provided herein can be prokaryotic cells or eukaryotic cells. Examples of prokaryotic cells that can be designed to include a nucleic acid provided herein include, without Attorney Docket No.48881-0047WO1 / 05958 limitation, E. coli (e.g., Tb-1, TG-1, DH5?, XL-Blue MRF (Stratagene), SA2821, or Y1090 cells), Bacillus subtilis, Salmonella typhimurium, Serratia marcescens, or Pseudomonas (e.g., P. aerugenosa) cells. Examples of eukaryotic cells that can be designed to include a nucleic acid provided herein include, without limitation, insect cells (e.g., Sf9 or Ea4 cells), yeast cells (e.g., S. cerevisiae cells), and mammalian cells (e.g., mouse, rat, hamster, monkey, or human cells). For example, VERO cells, HeLa cells, 3T3 cells, Chinese hamster ovary (CHO) cells, W138 BHK cells, COS-7 cells, and MDCK cells can be designed to include a nucleic acid provided herein. Any appropriate method can be used to introduce one or more nucleic acids provided herein (e.g., a vector such as a plasmid vector or viral vector having a nucleic acid sequence encoding at least part of a binder provided herein) into a host cell. For example, calcium chloride- mediated transformation, transduction, conjugation, triparental mating, DEAE, dextran- mediated transfection, infection, membrane fusion with liposomes, high velocity bombardment with DNA-coated microprojectiles, direct microinjection into single cells, electroporation, or combinations thereof can be used to introduce a nucleic acid provided herein into a host cell (see, e.g., Sambrook et al., Molecular Biology: A Laboratory Manual, Cold Spring Harbor Laboratory, NY (1989); Davis et al., Basic Methods in Molecular Biology (1986); and Neumann et al., EMBO J., 1:841 (1982)). In some cases, cells such as T cells, stem cells (e.g., induced pluripotent stem cells or mesenchymal stem cells), or NK cells can be designed to express one or more nucleic acids encoding a CAR described herein. For example, a population of T cells can be infected with viral vectors designed to express nucleic acid encoding a CAR described herein (e.g., a CAR having the ability to bind to a follistatin polypeptide). In some cases, cells such as T cells, stem cells (e.g., induced pluripotent stem cells or mesenchymal stem cells), or NK cells can be designed to express one or more nucleic acids encoding a cell engager described herein. For example, a population of T cells can be infected with viral vectors designed to express nucleic acid encoding a cell engager described herein (e.g., a cell engager having the ability to bind to a follistatin polypeptide). Attorney Docket No.48881-0047WO1 / 05958 In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein can be produced using a method that includes (a) introducing nucleic acid encoding the polypeptide into a host cell; (b) culturing the host cell in culture medium under conditions sufficient to express the polypeptide; (c) harvesting the polypeptide from the cell or culture medium; and (d) purifying the polypeptide (e.g., to reach at least 50, 60, 70, 80, 90, 95, 97, 98, or 99 percent purity). In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, cell engager, and / or ADC) provided herein, a nucleic acid provided herein (e.g., nucleic acid encoding an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager provided herein), a vector provided herein (e.g., a viral vector designed to express an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager provided herein), and / or a host cell provided herein (e.g., a host cell designed to express an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager provided herein) can be formulated as a pharmaceutical composition for administration to a mammal (e.g., a human) having cancer to treat that mammal. In some cases, a binder (e.g., an antibody, antigen binding fragment, antibody domain, cell engager, and / or ADC) provided herein, a nucleic acid provided herein (e.g., nucleic acid encoding an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager provided herein), a vector provided herein (e.g., a viral vector designed to express an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager provided herein), and / or a host cell provided herein (e.g., a host cell designed to express an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager provided herein) can be formulated as a pharmaceutical composition for administration to a mammal (e.g., a human) to reduce the number of cancer cells within the mammal and / or to increase the survival of the mammal suffering from cancer. For example, a binder (e.g., an antibody, antigen binding fragment, antibody domain, cell engager, and / or ADC) provided herein having the ability to bind to a follistatin polypeptide (e.g., a human follistatin polypeptide) can be formulated as a pharmaceutical composition for administration to a mammal (e.g., a human). In some cases, a Attorney Docket No.48881-0047WO1 / 05958 pharmaceutical composition provided herein can include a pharmaceutically acceptable carrier such as a buffer, a salt, a surfactant, a sugar, a tonicity modifier, or combinations thereof as, for example, described elsewhere (Gervasi et al., Eur. J. Pharmaceutics and Biopharmaceutics, 131:8-24 (2018)). Examples of pharmaceutically acceptable carriers that can be used to make a pharmaceutical composition provided herein include, without limitation, water, lactic acid, citric acid, sodium chloride, sodium citrate, sodium succinate, sodium phosphate, a surfactant (e.g., polysorbate 20, polysorbate 80, or poloxamer 188), dextran 40, or a sugar (e.g., sorbitol, mannitol, sucrose, dextrose, or trehalose), or combinations thereof. For example, a pharmaceutical composition designed to include a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, cell engager, and / or ADC) provided herein (or a nucleic acid, a vector, or a host cell provided herein) can be formulated to include a buffer (e.g., an acetate, citrate, histidine, succinate, phosphate, or hydroxymethylaminomethane (Tris) buffer), a surfactant (e.g., polysorbate 20, polysorbate 80, or poloxamer 188), and a sugar such as sucrose. Other ingredients that can be included within a pharmaceutical composition provided herein include, without limitation, amino acids such as glycine or arginine, antioxidants such as ascorbic acid, methionine, or ethylenediaminetetraacetic acid (EDTA), anticancer agents such as enzalutamide, imanitib, gefitinib, erlotini, sunitinib, lapatinib, nilotinib, sorafenib, temsirolimus, everolimus, pazopanib, crizotinib, ruxolitinib, axitinib, bosutinib, cabozantinib, ponatinib, regorafenib, ibrutinib, trametinib, perifosine, bortezomib, carfilzomib, batimastat, ganetespib, obatoclax, navitoclax, taxol, paclitaxel, or bevacizumab, or combinations thereof. For example, a pharmaceutical composition provided herein can be formulated to include one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cells designed to express a CAR having the ability to bind to a follistatin polypeptide, one or more cell engagers, and / or one or more ADCs) provided herein in combination with one or more checkpoint inhibitors such as anti-PD-1 antibodies or PD-1 inhibitors (e.g., cemiplimab, nivolumab, pembrolizumab, JTX-4014, spartalizumab, camrelizumab, sintilimab, tislelizumab, toripalimab, dostarlimab, INCMGA00012, AMP-224, or AMP-514), anti-PD-L1 antibodies or PD-L1 inhibitors Attorney Docket No.48881-0047WO1 / 05958 (e.g., avelumab, durvalumab, atezolizumab, KN035, CK-301, AUNP12, CA-170, or BMS-986189), and / or anti-CTLA-4 antibodies (e.g., ipilimumab). In some cases, a pharmaceutical composition provided herein can be formulated to include one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cells designed to express a CAR having the ability to bind to a follistatin polypeptide, one or more cell engagers, and / or one or more ADCs) provided herein in combination with one or more anticancer agents. In some cases, one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cells designed to express a CAR having the ability to bind to a follistatin polypeptide, one or more cell engagers, and / or one or more ADCs) provided herein can be used in combination with one or more anticancer agents to treat a mammal (e.g., a human) having cancer. Examples of anticancer agents that can be used as described herein include, without limitation, taxol compounds, cisplatin, Monomethyl Auristatin E (MMAE), tubulin inhibitor DM4, active metabolite SN-38, and pyrrolobenzodiazepine (PDB) dimer. In some cases, when a pharmaceutical composition is formulated to include one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cells designed to express a CAR having the ability to bind to a follistatin polypeptide, one or more cell engagers, and / or one or more ADCs) provided herein, any appropriate concentration of the binder can be used. For example, a pharmaceutical composition provided herein can be formulated to be a liquid that includes from about 1 mg to about 500 mg (e.g., from about 1 mg to about 500 mg, from about 10 mg to about 500 mg, from about 50 mg to about 500 mg, from about 100 mg to about 500 mg, from about 0.5 mg to about 250 mg, from about 0.5 mg to about 150 mg, from about 0.5 mg to about 100 mg, from about 0.5 mg to about 50 mg, from about 1 mg to about 300 mg, from about 2 mg to about 200 mg, from about 10 mg to about 300 mg, from about 25 mg to about 300 mg, from about 50 mg to about 150 mg, or from about 150 mg to about 300 mg) of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR+cell population, cell engager, and / or ADC) provided herein per mL. In another example, a pharmaceutical composition provided herein can be Attorney Docket No.48881-0047WO1 / 05958 formulated to be a solid or semi-solid that includes from about 0.5 mg to about 500 mg (e.g., from about 1 mg to about 500 mg, from about 10 mg to about 500 mg, from about 50 mg to about 500 mg, from about 100 mg to about 500 mg, from about 0.5 mg to about 250 mg, from about 0.5 mg to about 150 mg, from about 0.5 mg to about 100 mg, from about 0.5 mg to about 50 mg, from about 1 mg to about 300 mg, from about 10 mg to about 300 mg, from about 25 mg to about 300 mg, from about 50 mg to about 150 mg, or from about 150 mg to about 300 mg) of a binder (e.g., an antibody, antigen binding fragment, antibody domain, cell engager, and / or ADC) provided herein. In some cases, a pharmaceutical composition containing a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein can be formulated as a dosage form with a titer of the binder being from about 1 x 105to about 1 x 1012(e.g., from about 1 x 105to about 1 x 1010, from about 1 x 105to about 1 x 108, from about 1 x 106to about 1 x 1012, from about 1 x 106to about 1 x 1012, from about 1 x 108to about 1 x 1012, from about 1 x 109to about 1 x 1012, from about 1 x 106to about 1 x 1011, or from about 1 x 107to about 1 x 1010). In some cases, when a pharmaceutical composition is formulated to include one or more nucleic acids (e.g., vectors such as viral vectors) encoding at least part of a binder (e.g., an antibody, antigen binding fragment, antibody domain, CAR, and / or cell engager) provided herein, any appropriate concentration of the nucleic acid can be used. For example, a pharmaceutical composition provided herein can be formulated to be a liquid that includes from about 0.5 mg to about 500 mg (e.g., from about 1 mg to about 500 mg, from about 10 mg to about 500 mg, from about 50 mg to about 500 mg, from about 100 mg to about 500 mg, from about 0.5 mg to about 250 mg, from about 0.5 mg to about 150 mg, from about 0.5 mg to about 100 mg, from about 0.5 mg to about 50 mg, from about 1 mg to about 300 mg, from about 2 mg to about 200 mg, from about 10 mg to about 300 mg, from about 25 mg to about 300 mg, from about 50 mg to about 150 mg, or from about 150 mg to about 300 mg) of a nucleic acid provided herein per mL. In another example, a pharmaceutical composition provided herein can be formulated to be a solid or semi-solid that includes from about 0.5 mg to about 500 mg (e.g., from about 1 mg to about 500 mg, from about 10 mg to about 500 mg, from about 50 mg to about 500 Attorney Docket No.48881-0047WO1 / 05958 mg, from about 100 mg to about 500 mg, from about 0.5 mg to about 250 mg, from about 0.5 mg to about 150 mg, from about 0.5 mg to about 100 mg, from about 0.5 mg to about 50 mg, from about 1 mg to about 300 mg, from about 10 mg to about 300 mg, from about 25 mg to about 300 mg, from about 50 mg to about 150 mg, or from about 150 mg to about 300 mg) of a nucleic acid provided herein. In some cases, a pharmaceutical composition designed to include a binder (e.g., an antibody, antigen binding fragment, antibody domain, cell engager, and / or ADC) provided herein can be formulated to include one or more agents capable of reducing aggregation of the binder when formulated. Examples of such agents that can be used as described herein include, without limitation, methionine, arginine, lysine, aspartic acid, glycine, glutamic acid, and combinations thereof. In some cases, one or more of these amino acids can be included within the formulation at a concentration from about 0.5 mM to about 145 mM (e.g., from about 1 mM to about 145 mM, from about 10 mM to about 145 mM, from about 100 mM to about 145 mM, from about 0.5 mM to about 125 mM, from about 0.5 mM to about 100 mM, from about 0.5 mM to about 75 mM, or from about 10 mM to about 100 mM). A pharmaceutical composition provided herein can be in any appropriate form. For example, a pharmaceutical composition provided herein can designed to be a liquid, a semi-solid, or a solid. In some cases, a pharmaceutical composition provided herein can be a liquid solution (e.g., an injectable and / or infusible solution), a dispersion, a suspension, a tablet, a pill, a powder, a microemulsion, a liposome, or a suppository. In some cases, a pharmaceutical composition provided herein can be lyophilized. In some cases, a pharmaceutical composition provided herein (e.g., a pharmaceutical composition that includes one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein can be formulated with a carrier or coating designed to protect against rapid release. For example, a pharmaceutical composition provided herein can be formulated as a controlled release formulation or as a regulated release formulation as described elsewhere (U.S. Patent Application Publication Nos. 2019 / 0241667; 2019 / 0233522; and 2019 / 0233498). Attorney Docket No.48881-0047WO1 / 05958 This document also provides methods for administering a composition (e.g., a pharmaceutical composition provided herein) containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, or host cell (e.g., CAR+cells) provided herein) to a mammal (e.g., a human). For example, a composition (e.g., a pharmaceutical composition provided herein) containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, and / or host cell (e.g., CAR+cells) provided herein) can be administered to a mammal (e.g., a human) having cancer to treat that mammal. In some cases, a composition (e.g., a pharmaceutical composition provided herein) containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, and / or host cell (e.g., CAR+cells) provided herein) can be administered to a mammal (e.g. a human) to reduce the number of cancer cells within the mammal and / or to increase the survival of the mammal suffering from cancer. Any appropriate cancer can be treated using a composition (e.g., a pharmaceutical composition provided herein) containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, or host cell (e.g., CAR+cells) provided herein). For example, a mammal (e.g., a human) having cancer can be treated by administering a composition (e.g., a pharmaceutical composition) containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein to that mammal. Examples of cancers that can be treated as described herein include, without limitation, ovarian cancer, lung cancer, prostate cancer, pancreatic cancer, renal cancer, head and neck cancer, liver cancer, cervical cancer, urothelial cancer, thymic epithelial tumors, and breast cancer. In some cases, a mammal (e.g., a human) having a follistatin+cancer (e.g., a follistatin+ Attorney Docket No.48881-0047WO1 / 05958 ovarian cancer, a follistatin+lung cancer, or a follistatin+prostate cancer) can be administered a composition (e.g., a pharmaceutical composition) containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein to treat that mammal (e.g., to reduce the number of cancer cells within the mammal). Any appropriate method can be used to administer a composition (e.g., a pharmaceutical composition) provided herein to a mammal (e.g., a human). For example, a composition provided herein (e.g., a pharmaceutical composition containing one or more binders provided herein such as one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs provided herein) can be administered to a mammal (e.g., a human) intravenously (e.g., via an intravenous injection or infusion), intratumorally (e.g., via an intratumoral injection), subcutaneously (e.g., via a subcutaneous injection), intraperitoneally (e.g., via an intraperitoneal injection), orally, via inhalation, or intramuscularly (e.g., via intramuscular injection). In some cases, the route and / or mode of administration of a composition (e.g., a pharmaceutical composition provided herein) can be adjusted for the mammal being treated. In some cases, an effective amount of a composition containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, or host cell (e.g., CAR+cells) provided herein) (e.g., a pharmaceutical composition provided herein) can be an amount that reduces the number of cancer cells within a mammal having cancer without producing significant toxicity to the mammal. In some cases, an effective amount of a composition containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, or host cell (e.g., CAR+cells) provided herein) (e.g., a pharmaceutical composition provided herein) can be an amount that increases the survival time of a mammal having cancer as compared to a control mammal having Attorney Docket No.48881-0047WO1 / 05958 comparable cancer and not treated with the composition. For example, an effective amount of a binder (e.g., an antibody, antigen binding fragment, antibody domain, cell engager, and / or ADC) provided herein can be from about 0.001 mg / kg to about 100 mg / kg (e.g., from about 0.001 mg / kg to about 90 mg / kg, from about 0.001 mg / kg to about 80 mg / kg, from about 0.001 mg / kg to about 70 mg / kg, from about 0.001 mg / kg to about 60 mg / kg, from about 0.001 mg / kg to about 50 mg / kg, from about 0.001 mg / kg to about 40 mg / kg, from about 0.001 mg / kg to about 30 mg / kg, from about 0.005 mg / kg to about 100 mg / kg, from about 0.01 mg / kg to about 100 mg / kg, from about 0.05 mg / kg to about 100 mg / kg, from about 0.1 mg / kg to about 100 mg / kg, from about 0.5 mg / kg to about 100 mg / kg, from about 1 mg / kg to about 100 mg / kg, from about 5 mg / kg to about 100 mg / kg, from about 0.01 mg / kg to about 25 mg / kg, from about 0.1 mg / kg to about 30 mg / kg, from about 0.15 mg / kg to about 25 mg / kg, from about 0.2 mg / kg to about 20 mg / kg, from about 0.5 mg / kg to about 20 mg / kg, from about 1 mg / kg to about 30 mg / kg, from about 1 mg / kg to about 25 mg / kg, from about 1 mg / kg to about 20 mg / kg, from about 2 mg / kg to about 20 mg / kg, from about 5 mg / kg to about 30 mg / kg, from about 10 mg / kg to about 30 mg / kg, from about 15 mg / kg to about 30 mg / kg, from about 20 mg / kg to about 30 mg / kg, from about 3 mg / kg to about 30 mg / kg, from about 0.5 mg / kg to about 10 mg / kg, from about 1 mg / kg to about 10 mg / kg, from about 1 mg / kg to about 5 mg / kg, or from about 1 mg / kg to about 3 mg / kg). The effective amount can remain constant or can be adjusted as a sliding scale or variable dose depending on the mammal’s response to treatment. Various factors can influence the actual effective amount used for a particular application. For example, the severity of cancer when treating a mammal having cancer, the route of administration, the age and general health condition of the mammal, excipient usage, the possibility of co-usage with other therapeutic or prophylactic treatments such as use of other agents (e.g., checkpoint inhibitors), and the judgment of the treating physician may require an increase or decrease in the actual effective amount of a composition provided herein (e.g., a pharmaceutical composition containing one or more binders provided herein) that is administered. Attorney Docket No.48881-0047WO1 / 05958 In some cases, an effective frequency of administration of a composition containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, or host cell (e.g., CAR+cells) provided herein) (e.g., a pharmaceutical composition provided herein) can be a frequency that reduces the number of cancer cells within a mammal having cancer without producing significant toxicity to the mammal. In some cases, an effective frequency of administration of a composition containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, or host cell (e.g., CAR+cells) provided herein) (e.g., a pharmaceutical composition provided herein) can be a frequency that increases the survival time of a mammal having cancer as compared to a control mammal having comparable cancer and not treated with the composition. For example, an effective frequency of administration of a pharmaceutical composition provided herein such as a pharmaceutical composition containing one or more binders provided herein can be from about twice daily to about once a year (e.g., from about twice daily to about once a month, from about twice daily to about once a week, from about once daily to about once a month, or from one once daily to about once a week). In some cases, the frequency of administration of a pharmaceutical composition provided herein such as a pharmaceutical composition containing one or more binders provided herein can be daily. The frequency of administration of a pharmaceutical composition provided herein such as a pharmaceutical composition containing one or more binders provided herein can remain constant or can be variable during the duration of treatment. Various factors can influence the actual effective frequency used for a particular application. For example, the severity of the cancer, the route of administration, the age and general health condition of the mammal, excipient usage, the possibility of co-usage with other therapeutic or prophylactic treatments such as use of other agents (e.g., checkpoint inhibitors), and the judgment of the treating physician may require an increase or decrease in the actual effective Attorney Docket No.48881-0047WO1 / 05958 frequency of administration of a composition provided herein (e.g., a pharmaceutical composition containing one or more binders provided herein). In some cases, an effective duration of administration of a composition containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, or host cell (e.g., CAR+cells) provided herein) (e.g., a pharmaceutical composition provided herein) can be a duration that reduces the number of cancer cells within a mammal without producing significant toxicity to the mammal. In some cases, an effective duration of administration of a composition containing one or more binders (e.g., one or more antibodies, one or more antigen binding fragments, one or more antibody domains, one or more cell engagers, and / or one or more ADCs) provided herein (or a nucleic acid, vector, or host cell (e.g., CAR+cells) provided herein) (e.g., a pharmaceutical composition provided herein) can be a duration that increases the survival time of a mammal having cancer as compared to a control mammal having comparable cancer and not treated with the composition. For example, an effective duration of administration of a pharmaceutical composition provided herein such as a pharmaceutical composition containing one or more binders provided herein can vary from a single time point of administration to several weeks to several months (e.g., 4 to 12 weeks). Multiple factors can influence the actual effective duration used for a particular application. For example, the severity of the cancer, the route of administration, the age and general health condition of the mammal, excipient usage, the possibility of co-usage with other therapeutic or prophylactic treatments such as use of other agents (e.g., checkpoint inhibitors), and the judgment of the treating physician may require an increase or decrease in the actual effective duration of administration of a composition provided herein (e.g., a pharmaceutical composition containing one or more binders provided herein). In some cases, a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein can be used to detect the presence or absence of a follistatin polypeptide (e.g., a human follistatin polypeptide) in vitro, in situ, or in vivo (e.g., in vivo imaging within a mammal such as a human). For example, a binder (e.g., an Attorney Docket No.48881-0047WO1 / 05958 antibody, antigen binding fragment, and / or antibody domain) provided herein can be designed to include a label (e.g., a covalently attached radioactive, enzymatic, colorimetric, or fluorescent label). The labelled binder can be used to detect the presence or absence of a follistatin polypeptide (e.g., a human follistatin polypeptide) within a biological sample in vitro. Examples of biological samples that can be assessed using a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein include, without limitation, serum samples, plasma samples, tissue samples, biopsy samples, cell line samples, and tissue culture samples. In some cases, a biological sample that can be assessed as described herein can include mammalian body tissues and / or cells such as leukocytes, ovary tissue or cells, prostate tissue or cells, heart tissue or cells, placenta tissue or cells, pancreas tissue or cells, liver tissue or cells, spleen tissue or cells, lung tissue or cells, breast tissue or cells, head and neck tissue or cells, endometrium tissue or cells, colon tissue or cells, colorectal tissue or cells, cervix tissue or cells, stomach tissue or cells, or umbilical tissue or cells that may express a follistatin polypeptide (e.g., a human follistatin polypeptide). In some cases, a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein can be immobilized, e.g., on a support, and retention of a follistatin polypeptide (e.g., a human follistatin polypeptide) from a biological sample on the support can be detected, and / or vice versa. In some cases, a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein can be used in applications such as fluorescence polarization, microscopy, ELISA, centrifugation, chromatography, and / or cell sorting (e.g., fluorescence activated cell sorting). In some cases, a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein containing a label (e.g., a covalently attached radioactive label) can be used to detect the presence or absence of a follistatin polypeptide (e.g., a human follistatin polypeptide) within a mammal (e.g., a human). For example, a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein that is labelled (e.g., covalently labelled) with a radiolabel or an MRI detectable label can be administered to a mammal (e.g., a human), and that mammal can be assessed using a means for detecting the detectable label. In some cases, a mammal Attorney Docket No.48881-0047WO1 / 05958 can be scanned to evaluate the location(s) of a labelled binder provided herein within the mammal. For example, the mammal can be imaged using NMR or other tomographic techniques. Examples of labels that can be attached (e.g., covalently or non-covalently attached) to a binder (e.g., an antibody, antigen binding fragment, and / or antibody domain) provided herein include, without limitation, radiolabels such as131I,111In,123I,99mTc,32P,33P,125I,3H,14C, and188Rh, fluorescent labels such as fluorescein and rhodamine, nuclear magnetic resonance active labels, positron emitting isotopes detectable by a positron emission tomography (“PET”) scanner, chemiluminescers such as luciferin, and enzymatic markers such as a peroxidase or a phosphatase. In some cases, short-range radiation emitters such as isotopes detectable by short-range detector probes can be used. The invention will be further described in the following examples, which do not limit the scope of the invention described in the claims. EXAMPLES Example 1 – Obtaining binders having the ability to bind to a human follistatin polypeptide Large phage displayed antibody domain libraries were panned and screened to identify binders that bind to a human follistatin polypeptide using the amino acid sequence set forth in SEQ ID NO:113 (Example 2). To identify such binders, a polypeptide having amino acid residues 1 to 334 of human follistatin polypeptide set forth in Example 2 was fused to the AviTag / His tag / Fc tag sequence at the C-terminus of the follistatin sequence, and the follistatin-AviTag polypeptide was used for panning of human VH domain phage-displayed libraries. 14 VH domains (Clones #1-#14; Examples 3-16) were identified. The 14 VH domains (Clones #1-#14) were assessed for binding affinity and specificity to a human follistatin polypeptide using an ELISA (Figure 4). Clones #1-#14 exhibited high affinity binding to a human follistatin polypeptide having EC50(nM) values of 17, 19, 2.5, 18, 3, 2.5, 3.5, 3.2, 12.3, 0.5, 21, 21, 68, and 23, respectively. 12 of Attorney Docket No.48881-0047WO1 / 05958 the 14 VH domains were used to create Fc versions of the binders and assessed for binding to a human follistatin polypeptide using an ELISA (Figure 5). 12 of the 14 VH domains were assessed for binding to BSA (Figure 6). FST3G9 and FST1E8 were used to test if anti-follistatin binders can enhance the cancer cell killing by chemotherapeutic agents such as cisplatin. Both FST3G9 and FST1E8 enhanced the ability of cisplatin to kill cancer cells (Figure 7). See, also, Figures 8-9. Example 2 – Exemplary human follistatin sequences Human follistatin (isoform 1): MVRARHQPGGLCLLLLLLCQFMEDRSAQAGNCWLRQAKNGRCQVLYKTELSK EECCSTGRLSTSWTEEDVNDNTLFKWMIFNGGAPNCIPCKETCENVDCGPGKKC RMNKKNKPRCVCAPDCSNITWKGPVCGLDGKTYRNECALLKARCKEQPELEVQ YQGRCKKTCRDVFCPGSSTCVVDQTNNAYCVTCNRICPEPASSEQYLCGNDGVT YSSACHLRKATCLLGRSIGLAYEGKCIKAKSCEDIQCTGGKKCLWDFKVGRGRC SLCDELCPDSKSDEPVCASDNATYASECAMKEAACSSGVLLEVKHSGSCNSISED TEEEEEDEDQDYSFPISSILEW (SEQ ID NO:113) Nucleic acid encoding human follistatin (isoform 1): ATGGTCCGCGCGAGGCACCAGCCGGGTGGGCTTTGCCTCCTGCTGCTGCTGCT CTGCCAGTTCATGGAGGACCGCAGTGCCCAGGCTGGGAACTGCTGGCTCCGT CAAGCGAAGAACGGCCGCTGCCAGGTCCTGTACAAGACCGAACTGAGCAAG GAGGAGTGCTGCAGCACCGGCCGGCTGAGCACCTCGTGGACCGAGGAGGAC GTGAATGACAACACACTCTTCAAGTGGATGATTTTCAACGGGGGCGCCCCCA ACTGCATCCCCTGTAAAGAAACGTGTGAGAACGTGGACTGTGGACCTGGGAA AAAATGCCGAATGAACAAGAAGAACAAACCCCGCTGCGTCTGCGCCCCGGA TTGTTCCAACATCACCTGGAAGGGTCCAGTCTGCGGGCTGGATGGGAAAACC TACCGCAATGAATGTGCACTCCTAAAGGCAAGATGTAAAGAGCAGCCAGAAC TGGAAGTCCAGTACCAAGGCAGATGTAAAAAGACTTGTCGGGATGTTTTCTG TCCAGGCAGCTCCACATGTGTGGTGGACCAGACCAATAATGCCTACTGTGTG ACCTGTAATCGGATTTGCCCAGAGCCTGCTTCCTCTGAGCAATATCTCTGTGG GAATGATGGAGTCACCTACTCCAGTGCCTGCCACCTGAGAAAGGCTACCTGC CTGCTGGGCAGATCTATTGGATTAGCCTATGAGGGAAAGTGTATCAAAGCAA AGTCCTGTGAAGATATCCAGTGCACTGGTGGGAAAAAATGTTTATGGGATTT CAAGGTTGGGAGAGGCCGGTGTTCCCTCTGTGATGAGCTGTGCCCTGACAGT AAGTCGGATGAGCCTGTCTGTGCCAGTGACAATGCCACTTATGCCAGCGAGT GTGCCATGAAGGAAGCTGCCTGCTCCTCAGGTGTGCTACTGGAAGTAAAGCA CTCCGGATCTTGCAACTCCATTTCGGAAGACACCGAGGAAGAGGAGGAAGAT GAAGACCAGGACTACAGCTTTCCTATATCTTCTATTCTAGAGTGGTAA (SEQ ID NO:114) Attorney Docket No.48881-0047WO1 / 05958 Human follistatin (isoform 2): MVRARHQPGGLCLLLLLLCQFMEDRSAQAGNCWLRQAKNGRCQVLYKTELSK EECCSTGRLSTSWTEEDVNDNTLFKWMIFNGGAPNCIPCKETCENVDCGPGKKC RMNKKNKPRCVCAPDCSNITWKGPVCGLDGKTYRNECALLKARCKEQPELEVQ YQGRCKKTCRDVFCPGSSTCVVDQTNNAYCVTCNRICPEPASSEQYLCGNDGVT YSSACHLRKATCLLGRSIGLAYEGKCIKAKSCEDIQCTGGKKCLWDFKVGRGRC SLCDELCPDSKSDEPVCASDNATYASECAMKEAACSSGVLLEVKHSGSCN (SEQ ID NO:115) Nucleic acid encoding human follistatin (isoform 2): ATGGTCCGCGCGAGGCACCAGCCGGGTGGGCTTTGCCTCCTGCTGCTGCTGCT CTGCCAGTTCATGGAGGACCGCAGTGCCCAGGCTGGGAACTGCTGGCTCCGT CAAGCGAAGAACGGCCGCTGCCAGGTCCTGTACAAGACCGAACTGAGCAAG GAGGAGTGCTGCAGCACCGGCCGGCTGAGCACCTCGTGGACCGAGGAGGAC GTGAATGACAACACACTCTTCAAGTGGATGATTTTCAACGGGGGCGCCCCCA ACTGCATCCCCTGTAAAGAAACGTGTGAGAACGTGGACTGTGGACCTGGGAA AAAATGCCGAATGAACAAGAAGAACAAACCCCGCTGCGTCTGCGCCCCGGA TTGTTCCAACATCACCTGGAAGGGTCCAGTCTGCGGGCTGGATGGGAAAACC TACCGCAATGAATGTGCACTCCTAAAGGCAAGATGTAAAGAGCAGCCAGAAC TGGAAGTCCAGTACCAAGGCAGATGTAAAAAGACTTGTCGGGATGTTTTCTG TCCAGGCAGCTCCACATGTGTGGTGGACCAGACCAATAATGCCTACTGTGTG ACCTGTAATCGGATTTGCCCAGAGCCTGCTTCCTCTGAGCAATATCTCTGTGG GAATGATGGAGTCACCTACTCCAGTGCCTGCCACCTGAGAAAGGCTACCTGC CTGCTGGGCAGATCTATTGGATTAGCCTATGAGGGAAAGTGTATCAAAGCAA AGTCCTGTGAAGATATCCAGTGCACTGGTGGGAAAAAATGTTTATGGGATTT CAAGGTTGGGAGAGGCCGGTGTTCCCTCTGTGATGAGCTGTGCCCTGACAGT AAGTCGGATGAGCCTGTCTGTGCCAGTGACAATGCCACTTATGCCAGCGAGT GTGCCATGAAGGAAGCTGCCTGCTCCTCAGGTGTGCTACTGGAAGTAAAGCA CTCCGGATCTTGCAACTAA (SEQ ID NO:116) Example 3 – Sequences of exemplary anti-follistatin Clone #1 Anti-follistatin Clone #1 (also referred to as FST1) (VH Domain; capable of binding to follistatin) EVQLVESGGGVVQPGRSLRLSCAASGFTFSSYAMHWVRQAPGKGLEWVANIKH DGSDKYYADSVKGRFTISRDDSTNTLYLQMNSLRAEDTALYHCARGGPIVGAKP IREPRVVLGPGTLVTVSS (SEQ ID NO:8) Framework Region 1 of heavy chain variable domain: EVQLVESGGGVVQPGRSLRLSCAAS (SEQ ID NO:4) CDR1 of heavy chain variable domain: Attorney Docket No.48881-0047WO1 / 05958 GFTFSSYA (SEQ ID NO:1) Framework Region 2 of heavy chain variable domain: MHWVRQAPGKGLEWVAN (SEQ ID NO:5) CDR2 of heavy chain variable domain: IKHDGSDK (SEQ ID NO:2) Framework Region 3 of heavy chain variable domain: YYADSVKGRFTISRDDSTNTLYLQMNSLRAEDTALYHC (SEQ ID NO:6) CDR3 of heavy chain variable domain: ARGGPIVGAKPIREPRVV (SEQ ID NO:3) Framework Region 4 of heavy chain variable domain: LGPGTLVTVSS (SEQ ID NO:7) Example 4 – Sequences of exemplary anti-follistatin Clone #2 Anti-follistatin Clone #2 (also referred to as FST2) (VH Domain; capable of binding to follistatin) EVQLVESGGGVVQPGRSLRLSCAASGFTFSSYAMHWVRQAPGKGLEWIGEINHS GSTNYNPSLKSRVTVSRDNAKNTLYLQMNSLRAEDTALYYCARGALTVPWMLG PGTLVTVSS (SEQ ID NO:16) Framework Region 1 of light chain variable domain: EVQLVESGGGVVQPGRSLRLSCAAS (SEQ ID NO:12) CDR1 of light chai...

Claims

Attorney Docket No.48881-0047WO1 / 05958 WHAT IS CLAIMED IS:

1. An antibody comprising: (i) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:1 (or SEQ ID NO:1 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:2 (or SEQ ID NO:2 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:3 (or SEQ ID NO:3 with one, two, or three amino acid additions, deletions, or substitutions); (ii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:9 (or SEQ ID NO:9 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:10 (or SEQ ID NO:10 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:11 (or SEQ ID NO:11 with one, two, or three amino acid additions, deletions, or substitutions); (iii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:17 (or SEQ ID NO:17 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:18 (or SEQ ID NO:18 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:19 (or SEQ ID NO:19 with one, two, or three amino acid additions, deletions, or substitutions); (iv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:25 (or SEQ ID NO:25 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:26 (or SEQ ID NO:26 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:27 (or SEQ ID NO:27 with one, two, or three amino acid additions, deletions, or substitutions); (v) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:33 (or SEQ ID NO:33 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:34 (or SEQ ID NO:34 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:35 (or SEQ ID NO:35 with one, two, or three amino acid additions, deletions, or substitutions); (vi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:41 (or SEQ ID NO:41 with one, two, or three amino acidAttorney Docket No.48881-0047WO1 / 05958 additions, deletions, or substitutions), SEQ ID NO:42 (or SEQ ID NO:42 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:43 (or SEQ ID NO:43 with one, two, or three amino acid additions, deletions, or substitutions); (vii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:49 (or SEQ ID NO:49 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:50 (or SEQ ID NO:50 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:51 (or SEQ ID NO:51 with one, two, or three amino acid additions, deletions, or substitutions); (viii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:57 (or SEQ ID NO:57 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:58 (or SEQ ID NO:58 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:59 (or SEQ ID NO:59 with one, two, or three amino acid additions, deletions, or substitutions); (ix) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:65 (or SEQ ID NO:65 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:66 (or SEQ ID NO:66 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:67 (or SEQ ID NO:67 with one, two, or three amino acid additions, deletions, or substitutions); (x) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:73 (or SEQ ID NO:73 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:74 (or SEQ ID NO:74 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:75 (or SEQ ID NO:75 with one, two, or three amino acid additions, deletions, or substitutions); (xi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:81 (or SEQ ID NO:81 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:82 (or SEQ ID NO:82 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:83 (or SEQ ID NO:83 with one, two, or three amino acid additions, deletions, or substitutions); (xii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:89 (or SEQ ID NO:89 with one, two, or three aminoAttorney Docket No.48881-0047WO1 / 05958 acid additions, deletions, or substitutions), SEQ ID NO:90 (or SEQ ID NO:90 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:91 (or SEQ ID NO:91 with one, two, or three amino acid additions, deletions, or substitutions); (xiii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:97 (or SEQ ID NO:97 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:98 (or SEQ ID NO:98 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:99 (or SEQ ID NO:99 with one, two, or three amino acid additions, deletions, or substitutions); or (xiv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:105 (or SEQ ID NO:105 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:106 (or SEQ ID NO:106 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:107 (or SEQ ID NO:107 with one, two, or three amino acid additions, deletions, or substitutions).

2. The antibody of claim 1, wherein said antibody comprises the ability to bind to SEQ ID NO:113 or SEQ ID NO:

115.

3. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (i).

4. The antibody of claim 3, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

8.

5. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (ii).Attorney Docket No.48881-0047WO1 / 05958 6. The antibody of claim 5, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

16.

7. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (iii).

8. The antibody of claim 7, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

24.

9. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (iv).

10. The antibody of claim 9, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

32.

11. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (v).

12. The antibody of claim 11, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

40.

13. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (vi).Attorney Docket No.48881-0047WO1 / 05958 14. The antibody of claim 13, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

48.

15. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (vii).

16. The antibody of claim 15, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

56.

17. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (viii).

18. The antibody of claim 17, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

64.

19. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (ix).

20. The antibody of claim 19, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

72.

21. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (x).Attorney Docket No.48881-0047WO1 / 05958 22. The antibody of claim 21, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

80.

23. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (xi).

24. The antibody of claim 23, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

88.

25. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (xii).

26. The antibody of claim 25, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

96.

27. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (xiii).

28. The antibody of claim 27, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

104.

29. The antibody of any one of claims 1-2, wherein said antibody comprises said heavy chain variable domain or region of said (xiv).Attorney Docket No.48881-0047WO1 / 05958 30. The antibody of claim 29, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

112.

31. The antibody of any one of claims 1-30, wherein said antibody is a monoclonal antibody.

32. The antibody of any one of claims 1-31, wherein said antibody is an scFv antibody.

33. An antigen binding fragment comprising: (i) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:1 (or SEQ ID NO:1 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:2 (or SEQ ID NO:2 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:3 (or SEQ ID NO:3 with one, two, or three amino acid additions, deletions, or substitutions); (ii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:9 (or SEQ ID NO:9 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:10 (or SEQ ID NO:10 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:11 (or SEQ ID NO:11 with one, two, or three amino acid additions, deletions, or substitutions); (iii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:17 (or SEQ ID NO:17 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:18 (or SEQ ID NO:18 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:19 (or SEQ ID NO:19 with one, two, or three amino acid additions, deletions, or substitutions); (iv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:25 (or SEQ ID NO:25 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:26 (or SEQ ID NO:26 with one, two,Attorney Docket No.48881-0047WO1 / 05958 or three amino acid additions, deletions, or substitutions), and SEQ ID NO:27 (or SEQ ID NO:27 with one, two, or three amino acid additions, deletions, or substitutions); (v) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:33 (or SEQ ID NO:33 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:34 (or SEQ ID NO:34 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:35 (or SEQ ID NO:35 with one, two, or three amino acid additions, deletions, or substitutions); (vi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:41 (or SEQ ID NO:41 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:42 (or SEQ ID NO:42 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:43 (or SEQ ID NO:43 with one, two, or three amino acid additions, deletions, or substitutions); (vii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:49 (or SEQ ID NO:49 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:50 (or SEQ ID NO:50 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:51 (or SEQ ID NO:51 with one, two, or three amino acid additions, deletions, or substitutions); (viii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:57 (or SEQ ID NO:57 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:58 (or SEQ ID NO:58 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:59 (or SEQ ID NO:59 with one, two, or three amino acid additions, deletions, or substitutions); (ix) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:65 (or SEQ ID NO:65 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:66 (or SEQ ID NO:66 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:67 (or SEQ ID NO:67 with one, two, or three amino acid additions, deletions, or substitutions); (x) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:73 (or SEQ ID NO:73 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:74 (or SEQ ID NO:74 with one, two,Attorney Docket No.48881-0047WO1 / 05958 or three amino acid additions, deletions, or substitutions), and SEQ ID NO:75 (or SEQ ID NO:75 with one, two, or three amino acid additions, deletions, or substitutions); (xi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:81 (or SEQ ID NO:81 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:82 (or SEQ ID NO:82 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:83 (or SEQ ID NO:83 with one, two, or three amino acid additions, deletions, or substitutions); (xii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:89 (or SEQ ID NO:89 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:90 (or SEQ ID NO:90 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:91 (or SEQ ID NO:91 with one, two, or three amino acid additions, deletions, or substitutions); (xiii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:97 (or SEQ ID NO:97 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:98 (or SEQ ID NO:98 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:99 (or SEQ ID NO:99 with one, two, or three amino acid additions, deletions, or substitutions); or (xiv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:105 (or SEQ ID NO:105 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:106 (or SEQ ID NO:106 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:107 (or SEQ ID NO:107 with one, two, or three amino acid additions, deletions, or substitutions).

34. The antigen binding fragment of claim 33, wherein said antigen binding fragment comprises the ability to bind to SEQ ID NO:113 or SEQ ID NO:

115.

35. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (i).Attorney Docket No.48881-0047WO1 / 05958 36. The antigen binding fragment of claim 35, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

8.

37. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (ii).

38. The antigen binding fragment of claim 37, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

16.

39. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (iii).

40. The antigen binding fragment of claim 39, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

24.

41. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (iv).

42. The antigen binding fragment of claim 41, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

32.

43. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (v).Attorney Docket No.48881-0047WO1 / 05958 44. The antigen binding fragment of claim 43, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

40.

45. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (vi).

46. The antigen binding fragment of claim 45, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

48.

47. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (vii).

48. The antigen binding fragment of claim 47, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

56.

48. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (viii).

49. The antigen binding fragment of claim 48, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

64.

50. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (ix).Attorney Docket No.48881-0047WO1 / 05958 51. The antigen binding fragment of claim 50, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

72.

52. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (x).

53. The antigen binding fragment of claim 52, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

80.

54. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (xi).

55. The antigen binding fragment of claim 54, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

88.

56. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (xii).

57. The antigen binding fragment of claim 56, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

96.

58. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (xiii).Attorney Docket No.48881-0047WO1 / 05958 59. The antigen binding fragment of claim 58, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

104.

60. The antigen binding fragment of any one of claims 33-34, wherein said antigen binding fragment comprises said heavy chain variable domain or region of said (xiv).

61. The antigen binding fragment of claim 60, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

112.

62. The antigen binding fragment of any one of claims 33-61, wherein said antigen binding fragment is monoclonal.

63. The antigen binding fragment of any one of claims 33-62, wherein said antigen binding fragment is a Fab.

64. An antibody domain comprising: (i) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:1 (or SEQ ID NO:1 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:2 (or SEQ ID NO:2 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:3 (or SEQ ID NO:3 with one, two, or three amino acid additions, deletions, or substitutions); (ii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:9 (or SEQ ID NO:9 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:10 (or SEQ ID NO:10 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:11 (or SEQ ID NO:11 with one, two, or three amino acid additions, deletions, or substitutions); (iii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:17 (or SEQ ID NO:17 with one, two, or three aminoAttorney Docket No.48881-0047WO1 / 05958 acid additions, deletions, or substitutions), SEQ ID NO:18 (or SEQ ID NO:18 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:19 (or SEQ ID NO:19 with one, two, or three amino acid additions, deletions, or substitutions); (iv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:25 (or SEQ ID NO:25 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:26 (or SEQ ID NO:26 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:27 (or SEQ ID NO:27 with one, two, or three amino acid additions, deletions, or substitutions); (v) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:33 (or SEQ ID NO:33 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:34 (or SEQ ID NO:34 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:35 (or SEQ ID NO:35 with one, two, or three amino acid additions, deletions, or substitutions); (vi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:41 (or SEQ ID NO:41 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:42 (or SEQ ID NO:42 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:43 (or SEQ ID NO:43 with one, two, or three amino acid additions, deletions, or substitutions); (vii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:49 (or SEQ ID NO:49 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:50 (or SEQ ID NO:50 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:51 (or SEQ ID NO:51 with one, two, or three amino acid additions, deletions, or substitutions); (viii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:57 (or SEQ ID NO:57 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:58 (or SEQ ID NO:58 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:59 (or SEQ ID NO:59 with one, two, or three amino acid additions, deletions, or substitutions); (ix) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:65 (or SEQ ID NO:65 with one, two, or three amino acidAttorney Docket No.48881-0047WO1 / 05958 additions, deletions, or substitutions), SEQ ID NO:66 (or SEQ ID NO:66 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:67 (or SEQ ID NO:67 with one, two, or three amino acid additions, deletions, or substitutions); (x) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:73 (or SEQ ID NO:73 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:74 (or SEQ ID NO:74 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:75 (or SEQ ID NO:75 with one, two, or three amino acid additions, deletions, or substitutions); (xi) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:81 (or SEQ ID NO:81 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:82 (or SEQ ID NO:82 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:83 (or SEQ ID NO:83 with one, two, or three amino acid additions, deletions, or substitutions); (xii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:89 (or SEQ ID NO:89 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:90 (or SEQ ID NO:90 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:91 (or SEQ ID NO:91 with one, two, or three amino acid additions, deletions, or substitutions); (xiii) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:97 (or SEQ ID NO:97 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:98 (or SEQ ID NO:98 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:99 (or SEQ ID NO:99 with one, two, or three amino acid additions, deletions, or substitutions); or (xiv) a heavy chain variable domain or region comprising the amino acid sequences set forth in SEQ ID NO:105 (or SEQ ID NO:105 with one, two, or three amino acid additions, deletions, or substitutions), SEQ ID NO:106 (or SEQ ID NO:106 with one, two, or three amino acid additions, deletions, or substitutions), and SEQ ID NO:107 (or SEQ ID NO:107 with one, two, or three amino acid additions, deletions, or substitutions).Attorney Docket No.48881-0047WO1 / 05958 65. The antibody domain of claim 64, wherein said antibody domain comprises the ability to bind to SEQ ID NO:113 or SEQ ID NO:

115.

66. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (i).

67. The antibody domain of claim 66, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

8.

68. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (ii).

69. The antibody domain of claim 68, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

16.

70. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (iii).

71. The antibody domain of claim 70, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

24.

72. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (iv).Attorney Docket No.48881-0047WO1 / 05958 73. The antibody domain of claim 72, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

32.

74. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (v).

75. The antibody domain of claim 74, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

40.

76. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (vi).

77. The antibody domain of claim 76, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

48.

78. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (vii).

79. The antibody domain of claim 78, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

56.

80. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (viii).Attorney Docket No.48881-0047WO1 / 05958 81. The antibody domain of claim 80, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

64.

82. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (ix).

83. The antibody domain of claim 82, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

72.

84. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (x).

85. The antibody domain of claim 84, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

80.

86. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (xi).

87. The antibody domain of claim 86, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

88.

88. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (xii).Attorney Docket No.48881-0047WO1 / 05958 89. The antibody domain of claim 88, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

96.

90. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (xiii).

91. The antibody domain of claim 90, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

104.

92. The antibody domain of any one of claims 64-65, wherein said antibody domain comprises said heavy chain variable domain or region of said (xiv).

93. The antibody domain of claim 92, wherein said heavy chain variable domain or region comprises an amino acid sequence having at least 90 percent identity to the amino acid sequence set forth in SEQ ID NO:

112.

94. The antibody domain of any one of claims 64-93, wherein said antibody domain is monoclonal.

95. The antibody domain of any one of claims 64-94, wherein said antibody domain is a VH domain.

96. A chimeric antigen receptor comprising an antigen binding domain, a hinge, a transmembrane domain, and one or more signaling domains, wherein said antigen binding domain comprises an antibody, an antigen-binding fragment, or an antibody domain of any one of claims 1-95.Attorney Docket No.48881-0047WO1 / 05958 97. The chimeric antigen receptor of claim 96, wherein said antigen binding domain comprises a VH domain having the ability to bind to a follistatin polypeptide.

98. The chimeric antigen receptor of any one of claims 96-97, wherein said hinge comprises a hinge set forth in Example 23.

99. The chimeric antigen receptor of any one of claims 96-98, wherein said transmembrane domain comprises a transmembrane domain set forth in Example 24.

100. The chimeric antigen receptor of any one of claims 96-99, wherein said chimeric antigen receptor comprises one or more signaling domains set forth in Example 25.

101. A cell comprising a chimeric antigen receptor of any one of claims 96-100.

102. The cell of claim 101, wherein said cell is a T cell, a stem cell, or an NK cell.

103. A cell engager comprising a first antigen binding domain, a linker, and a second antigen binding domain, wherein said first antigen binding domain comprises an antibody, an antigen-binding fragment, or an antibody domain of any one of claims 1-95.

104. The cell engager of claim 103, wherein said first antigen binding domain comprises a VH domain having the ability to bind to a follistatin polypeptide.

105. The cell engager of claim 103, wherein said first antigen binding domain is an IgG having the ability to bind to a follistatin polypeptide.

106. The cell engager of any one of claims 103-105, wherein said linker comprises a linker set forth in Example 20 or Example 23.Attorney Docket No.48881-0047WO1 / 05958 107. The cell engager of any one of claims 103-106, wherein said second antigen binding domain binds to a polypeptide expressed on the surface of T cells.

108. The cell engager of claim 107, wherein said polypeptide expressed on the surface of T cells is a CD3 polypeptide.

109. The cell engager of claim 107, wherein said second antigen binding domain is an antigen binding domain set forth in Example 28.

110. The cell engager of any one of claims 103-106, wherein said second antigen binding domain binds to a polypeptide expressed on the surface of NK cells.

111. The cell engager of claim 110, wherein said polypeptide expressed on the surface of NK cells is a CD16a, NKG2A, NKG2D, NKp30, NKp44, or NKp46 polypeptide.

112. The cell engager of claim 110, wherein said second antigen binding domain is an antigen binding domain set forth in Example 29.

113. The cell engager of any one of claims 103-112, wherein said cell engager comprises a third antigen binding domain.

114. The cell engager of claim 113, wherein said third antigen binding domain binds to a polypeptide expressed on the surface of NK cells.

115. The cell engager of claim 114, wherein said polypeptide expressed on the surface of NK cells is a CD16a, NKG2A, NKG2D, NKp30, NKp44, or NKp46 polypeptide.

116. The cell engager of claim 114, wherein said third antigen binding domain is an antigen binding domain set forth in Example 29.Attorney Docket No.48881-0047WO1 / 05958 117. A nucleic acid comprising a nucleic acid sequence encoding at least part of said antibody, said antigen-binding fragment, or said antibody domain of any one of claims 1- 95.

118. The nucleic acid of claim 117, wherein said nucleic acid sequence encodes said heavy chain variable domain or region of any one of said (i)-(xiv) of claim 1.

119. The nucleic acid of any one of claims 117-118, wherein said nucleic acid is a viral vector.

120. The nucleic acid of any one of claims 117-118, wherein said nucleic acid is a phagemid.

121. A nucleic acid comprising a nucleic acid sequence encoding a chimeric antigen receptor of any one of claims 96-100 or a cell engager of any one of claims 103-116.

122. The nucleic acid of claim 121, wherein said nucleic acid is a viral vector.

123. The nucleic acid of claim 121, wherein said nucleic acid is a phagemid.

124. A host cell comprising a nucleic acid of any one of claims 121-123.

125. A host cell that expresses a chimeric antigen receptor of any one of claims 96-100 or a cell engager of any one of claims 103-116.

126. The host cell of any one of claims 124-125, wherein said host cell is a T cell, stem cell, or NK cell.Attorney Docket No.48881-0047WO1 / 05958 127. An antibody-drug conjugate (ADC) comprising an antigen binging domain covalently linked to a drug, wherein said antigen binging domain comprises an antibody, an antigen binding fragment, or an antibody domain of any one of claims 1-95.

128. The ADC of claim 127, wherein said antigen binding domain comprises a VH domain having the ability to bind to a follistatin polypeptide.

129. The ADC of any one of claims 127-128, wherein said drug is selected from the group consisting of auristatins, mertansine, or pyrrolobenzodiazepine (PBD) dimers.

130. A composition comprising an antibody, an antigen binding fragment, or an antibody domain of any one of claims 1-95.

131. The composition of claim 130, wherein said composition comprises said antibody of any one of claims 1-32.

132. The composition of claim 130, wherein said composition comprises said antigen binding fragment of any one of claims 33-63.

133. The composition of claim 130, wherein said composition comprises said antibody domain of any one of claims 64-95.

134. A composition comprising a cell engager of any one of claims 103-116.

135. A composition comprising a cell of any one of claims 101-102 and 124-126.

136. A composition comprising an ADC of any one of claims 127-129.

137. The composition of any one of claims 130-136, wherein said composition comprises a checkpoint inhibitor.Attorney Docket No.48881-0047WO1 / 05958 138. The composition of claim 137, wherein said checkpoint inhibitor is selected from the group consisting of cemiplimab, nivolumab, pembrolizumab, JTX-4014, spartalizumab, camrelizumab, sintilimab, tislelizumab, toripalimab, dostarlimab, INCMGA00012, AMP-224, AMP-514, avelumab, durvalumab, atezolizumab, KN035, CK-301, AUNP12, CA-170, BMS-986189, and ipilimumab.

139. A method of treating a mammal having cancer, wherein said method comprises administering, to said mammal, a composition of any one of claims 130-138.

140. The method of claim 139, wherein said mammal is a human.

141. The method of any one of claims 139-140, wherein said cancer is a follistatin+cancer.

142. The method of claim 141, wherein said follistatin+cancer is selected from the group consisting of follistatin+ovarian cancer, follistatin+lung cancer, and follistatin+prostate cancer.

143. The method of any one of claims 139-142, wherein the number of cancer cells within said mammal is reduced following said administering step.

144. A method of treating a mammal having cancer, wherein said method comprises: (a) administering, to said mammal, said composition of any one of claims 130- 138, and (b) administering, to said mammal, a composition comprising a checkpoint inhibitor.

145. The method of claim 144, wherein said mammal is a human.Attorney Docket No.48881-0047WO1 / 05958 146. The method of any one of claims 144-145, wherein said cancer is a follistatin+cancer.

147. The method of claim 146, wherein said follistatin+cancer is selected from the group consisting of follistatin+ovarian cancer, follistatin+lung cancer, and follistatin+prostate cancer.

148. The method of any one of claims 144-147, wherein said checkpoint inhibitor is selected from the group consisting of cemiplimab, nivolumab, pembrolizumab, JTX- 4014, spartalizumab, camrelizumab, sintilimab, tislelizumab, toripalimab, dostarlimab, INCMGA00012, AMP-224, AMP-514, avelumab, durvalumab, atezolizumab, KN035, CK-301, AUNP12, CA-170, BMS-986189, and ipilimumab.

149. The method of any one of claims 144-148, wherein the number of cancer cells within said mammal is reduced following said administering steps (a) and (b).

150. A method for binding a binding molecule to a follistatin polypeptide, wherein said method comprises contacting said follistatin polypeptide with an antibody, an antigen binding fragment, or an antibody domain of any one of claims 1-95.

151. The method of claim 150, wherein said contacting is performed in vitro.

152. The method of claim 150, wherein said contacting is performed in vivo.

153. The method of claim 152, wherein said contacting is performed within a mammal by administering said antibody, said antigen binding fragment, or said antibody domain to said mammal.

154. The method of claim 153, wherein said mammal is a human.Attorney Docket No.48881-0047WO1 / 05958 155. A method for binding a binding molecule to a follistatin polypeptide, wherein said method comprises contacting said follistatin polypeptide with a chimeric antigen receptor of any one of claims 96-100, a cell engager of any one of claims 103-116, or an ADC of any one of claims 127-129.

156. The method of claim 155, wherein said contacting is performed in vitro.

157. The method of claim 155, wherein said contacting is performed in vivo.

158. The method of claim 157, wherein said contacting is performed within a mammal by administering said chimeric antigen receptor, said cell engager, or said ADC to said mammal.

159. The method of claim 158, wherein said mammal is a human.

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