The present invention provides, inter alia, compounds having the structures of formulas described herein; pharmaceutically acceptable salts, solvates, hydrates, tautomers, and isotopic forms thereof; and compositions (e.g., pharmaceutical compositions and kits) containing one or more of the foregoing. Also provided are methods of administering and uses involving the compounds and / or pharmaceutical compositions for treating or preventing disease. The disease can be a proliferative disease, such as a cancer (e.g., a blood cancer (e.g., a leukemia or lymphoma), a brain cancer, a breast cancer, melanoma, multiple myeloma, or an ovarian cancer) a benign neoplasm, pathologic angiogenesis, or a fibrotic disease. While no aspect of the invention is limited by the biological events that may transpire, administering a compound or other composition described herein may selectively inhibit the aberrant expression or activity of cyclin-dependent kinase 7 (CDK7) and, thereby, induce cellular apoptosis and / or inhibit the transcription of disease-related genes in the patient (or in a biological sample).