Nucleic acid construct capable of measuring homologous recombination activity and utilization thereof

Abstract

Disclosed are a means that enables simple, rapid detection of the existence and degree of HR activity in an individual and a means useful in the detection and treatment of homologous recombination repair cancers. The nucleic acid construct of the present invention includes a set of a first nucleic acid molecule and a second nucleic acid molecule. The first nucleic acid molecule includes a promoter region and, operably linked downstream thereof, a mutant gene sequence having a cleavage site in the interior of a gene sequence that encodes a protein, and the second nucleic acid molecule includes a complementary region constituting a first homologous region and a second homologous region capable of substituting by homologous recombination the partial region including the cleavage site in the mutant gene sequence. The construct is useful as a reagent or kit for measuring homologous recombination activity, as a diagnostic agent for homologous recombination deletion cancers, as a therapeutic agent for homologous recombination repair cancers, etc.

Description

Nucleic acid construct capable of measuring homologous recombination activity and use thereof The present invention relates to a nucleic acid construct capable of measuring homologous recombination activity, and use of the nucleic acid construct as a reagent or kit for measuring homologous recombination activity, an agent for detecting homologous recombination-deficient cancer, etc. Double-strand breaks in genomic DNA are the most mutagenic and dangerous form of DNA damage. Therefore, homologous recombination (HR), which can accurately repair double-strand breaks, is essential for maintaining genome stability (Non-Patent Document 1). HR occurs only during the S to G2 phases when sister chromatids exist as templates, but non-homologous end joining (NHEJ) is another double-strand break repair mechanism that functions regardless of the cell cycle (Non-Patent Document 2). In addition to these, there are minor repair pathways such as alternative end-joining and single-strand annealing, which involve PolQ, but in both cases, mutations are always induced after repair (Non-Patent Document 3). HR is a very complex reaction involving many proteins, and RAD51, which mediates strand exchange, plays a central role (Non-Patent Document 4). Factors that support the function of RAD51, such as BRCA1 and BRCA2, are also essential for HR, and human cells lacking BRCA1 or BRCA2 exhibit abnormalities in the repair of double-strand breaks by HR (Non-Patent Document 5). HR deficiency causes various cancers. Hereditary breast and ovarian cancer syndrome (HBOC) is the most common hereditary tumor, and its main cause is genetic abnormalities in BRCA1 or BRCA2 (Non-Patent Documents 6 and 7). Mutations in HR-related genes have also been reported in various types of sporadic (non-hereditary) cancers (Non-Patent Documents 8 and 9). Diagnosis of HR-deficient cancers is performed by genetic testing of BRCA1 and BRCA2 (Non-Patent Document 10), but considering that several dozen proteins are involved in the HR reaction, current genetic testing is not sufficient. It is considered more desirable to detect HR activity itself, and several methods have been proposed for this purpose (Non-Patent Documents 11, 12), but all of them are methods for evaluating HR activity relatively. It is considered most effective and reliable to evaluate the HR activity of individual cells as an absolute value rather than a relative evaluation, but such a method has not been developed. Platinum drugs or Poly(ADP-ribose)polymerase (PARP) inhibitors are used to treat HR-deficient cancers (Non-Patent Documents 13, 14). The latter PARP inhibitors in particular have attracted attention because of their "synthetic lethality" relationship with HR. However, a major problem with PARP inhibitor treatment is the frequent occurrence of resistant cancers, and there is currently no method to treat such resistant cancers (Non-Patent Documents 15, 16). Moynahan ME and Jasin M. Nat Rev Mol Cell Biol. 2010 Mar;11(3):196-207.Chang HHY et al. Nat Rev Mol Cell Biol. 2017 Aug;18(8):495-506.Saito S et al. Nat Commun. 2017 Jul 11;8:16112Morrical SW. Cold Spring Harb Perspect Biol. 2015 Feb 2;7(2):a016444.Prakash R et al. Cold Spring Harb Perspect Biol. 2015 Apr 1;7(4):a016600.King MC et al. Science. 2003 Oct 24;302(5645):643-646.Cancer Genome Atlas Research Network. Nature. 2011 Jun 29;474(7353):609-615.Heeke AL et al. JCO Precis Oncol. 2018;2018.Lord CJ and Ashworth A. Nat Rev Cancer. 2016 Feb;16(2):110-120.Gourley C et al. J Clin Oncol. 2019 Sep 1;37(25):2257-2269.Telli ML et al. Clin Cancer Res. 2016 Aug 1;22(15):3764-3773.Ransburgh DJ et al. Cancer Res. 2010 Feb 1;70(3):988-995.Bryant HE et al. Nature. 2005 Apr 14;434(7035):913-917.Farmer H et al. Nature. 2005 Apr 14;434(7035):917-921.Lord CJ and Ashworth A. Nat Med. 2013 Nov;19(11):1381-1388.Noordermeer SM and van Attikum H. Trends Cell Biol. 2019 Oct;29(10):820-834.Norquist B et al.J Clin Oncol. 2011 Aug 1;29(22):3008-3015.Bouwman P and Jonkers J. Clin Cancer Res. 2014 Feb 1;20(3):540-547.Pierce AJ et al. Genes Dev. 1999 Oct 15;13(20):2633-2638.Liang F et al. Proc Natl Acad Sci US A. 1996 Aug 20;93(17):8929-8933. If it becomes possible to easily and quickly detect the presence or absence and the degree of HR activity in cells, tissues, and individuals, it is expected that this method will be extremely useful for the diagnosis and treatment of HR-deficient cancers, including HBOC, regardless of the type of cancer. In addition, in the case of BRCA1-deficient cancers, since the restoration (reversion mutation) of BRCA1 is one of the factors that causes the acquisition of resistance to PARP inhibitors (Non-Patent Documents 17, 18), if cancer cells with restored HR activity can be detected and killed, this will have a major impact on the treatment of resistant cancers. However, there are still no known means for easily and quickly detecting the presence or absence and the degree of HR activity in individuals, or for selectively detecting and killing cancer cells with restored HR activity. An object of the present invention is to provide a means for easily and quickly detecting the presence or absence and the level of HR activity in an individual. Another object of the present invention is to provide a means useful for detecting and treating homologous recombination-restoring cancers for which no therapeutic means are currently available. As a result of intensive research, the inventors of the present application have developed a split-type nucleic acid construct composed of a set of two nucleic acid molecules as a nucleic acid construct with a structure designed so that in cells having homologous recombination activity, a gene sequence encoding a protein is reproduced and the protein is expressed, and in cells that have lost homologous recombination activity, the gene sequence is not reproduced and the protein is not expressed. They have also discovered that with this nucleic acid construct, the presence or absence of homologous recombination activity can be detected in a very short time in a transient expression system and that homologous recombination activity can be evaluated and measured in absolute values ​​rather than relative evaluation, and that if the nucleic acid construct of the present invention is prepared using a gene sequence encoding a protein that has the effect of reducing the survival rate of cells, such as a suicide gene, a therapeutic effect can be obtained by killing cancer cells that have recovered from homologous recombination, and have completed the present invention which includes the following aspects. [1] A nucleic acid construct comprising a set of a first nucleic acid molecule and a second nucleic acid molecule, The first nucleic acid molecule comprises a promoter region and a mutant gene sequence having a cleavage site within a gene sequence encoding a protein, the mutant gene sequence being located downstream of the promoter region; The second nucleic acid molecule is a nucleic acid construct comprising a complementary region composed of a first homologous region and a second homologous region capable of replacing a partial region including the cleavage site in the mutant gene sequence by homologous recombination, the complementary region comprising a base sequence selected from the following (i) to (iii): (i) A continuous subsequence in a gene sequence encoding the protein, the subsequence including the upstream and downstream regions adjacent to the cleavage site in the mutant gene sequence. (ii) A nucleotide sequence that is homologous to a partial sequence of (i) and encodes the same amino acid sequence as said partial sequence. (iii) A contiguous partial sequence in a gene sequence encoding a protein having 80% or more sequence identity with the protein and the same activity as the protein, said partial sequence having homology to the partial sequence of (i). [2] The nucleic acid construct according to [1], wherein the mutant gene sequence contains a stop codon upstream of the cleavage site. [3] The nucleic acid construct according to [1] or [2], wherein the base sequences of (ii) and (iii) have a homology of 90% or more to the partial sequence of (i). [4] The nucleic acid construct described in [1], wherein the second nucleic acid molecule is a continuous subsequence in a gene sequence encoding the protein, the subsequence including the upstream and downstream regions adjacent to the cleavage site in the mutant gene sequence. [5] The nucleic acid construct described in [2], wherein the second nucleic acid molecule is a continuous subsequence in a gene sequence encoding the protein, the subsequence including the upstream and downstream regions adjacent to a stop codon and a cleavage site in the mutant gene sequence. [6] The nucleic acid construct according to any one of [1] to [5], wherein the first homologous region and the second homologous region each have a length of at least 20 bases. [7] The nucleic acid construct according to any one of [1] to [6], wherein the first nucleic acid molecule comprises a poly A addition signal downstream of the mutant gene sequence. [8] The nucleic acid construct according to any one of [1] to [7], wherein the cleavage site is a restriction enzyme recognition site. [9] The nucleic acid construct according to any one of [1] to [8], wherein the second nucleic acid molecule is a circular nucleic acid molecule further comprising an additional sequence of 1 to 20,000 bases linked to the complementary region, or a linear nucleic acid molecule in which an additional sequence of 1 to 10,000 bases is linked to at least one of the 5' end and the 3' end of the complementary region.

[0010] The nucleic acid construct according to any one of [1] to [9], wherein the first nucleic acid molecule is a circular nucleic acid molecule or a linear nucleic acid molecule obtained by cleaving the circular nucleic acid molecule at the cleavage site.

[0011] The nucleic acid construct according to any one of claims [1] to

[0010] , wherein the gene sequence is a gene sequence encoding a protein having an effect of reducing cell viability, or a protein whose expression in a cell can be detected.

[0012] The nucleic acid construct described in

[0011] , wherein the gene sequence is the sequence of a suicide gene, a DNA damage-inducing gene, a DNA repair inhibitor gene, a luciferase gene, a fluorescent protein gene, a cell surface antigen gene, a secretory protein gene, or a membrane protein gene.

[0013] A reagent for measuring homologous recombination activity, comprising the nucleic acid construct according to any one of [1] to

[0012] , wherein the gene sequence is a gene sequence encoding a protein whose expression in a cell can be detected.

[0014]

[0013] A kit for measuring homologous recombination activity, comprising the measuring reagent described above.

[0015] A screening system for a drug that affects homologous recombination activity, comprising a nucleic acid construct described in any one of [1] to

[0012] .

[0016] A diagnostic agent for homologous recombination deficient cancer, comprising the nucleic acid construct described in any one of [1] to

[0012] .

[0017] A detection agent for detecting homologous recombination restored cancer cells, the detection agent comprising a nucleic acid construct described in any one of [1] to

[0012] , wherein the gene sequence is a gene sequence encoding a protein whose expression in a cell can be detected.

[0018] The detection agent described in

[0017] , wherein the homologous recombination restored cancer cells are PARP inhibitor resistant cancer cells.

[0019] A companion diagnostic agent for predicting the effect of an anticancer drug on a homologous recombination deficient cancer, the companion diagnostic agent comprising the nucleic acid construct according to any one of [1] to

[0012] .

[0020] The companion diagnostic agent of

[0019] , wherein the anticancer drug is a DNA damaging anticancer drug.

[0021] The companion diagnostic agent of

[0020] , wherein the DNA damaging anticancer drug is a PARP inhibitor.

[0022] A therapeutic agent for homologous recombination restored cancer, comprising the nucleic acid construct according to any one of [1] to

[0012] , wherein the gene sequence is a gene sequence encoding a protein having an effect of reducing cell viability.

[0023] The therapeutic agent described in

[0022] , wherein the homologous recombination restored cancer is a PARP inhibitor resistant cancer.

[0024] introducing the nucleic acid construct according to any one of [1] to

[0012] , wherein the gene sequence is a gene sequence encoding a protein whose expression in a cell can be detected, into a test cell in which homologous recombination activity is to be measured and into a homologous recombination-non-deficient control cell having normal homologous recombination activity; Measuring the expression level of the protein in the test cells and in a control cell that is not deficient in homologous recombination; and Comparing the expression level in the test cells with the expression level in control cells that are not deficient in homologous recombination A method for measuring homologous recombination activity in a test cell, comprising:

[0025] Introducing the nucleic acid construct according to any one of [1] to

[0012] into a cell having normal homologous recombination activity, and then treating the cell with an individual compound from the compound group; or treating a cell having normal homologous recombination activity with an individual compound from the compound group, and then introducing the nucleic acid construct according to any one of [1] to

[0012] into the cell; and Measuring the expression of said protein. A method for identifying candidate drugs that affect homologous recombination activity, comprising:

[0026] The method described in

[0025] , wherein the protein is a protein whose expression in a cell can be detected as a signal, and when a lower protein signal is detected in a cell treated with the compound than in a cell not treated with the compound, the method comprises selecting the compound as a candidate inhibitor that inhibits homologous recombination activity, or when a faster onset of the protein signal or a higher signal is detected in a cell treated with the compound than in a cell not treated with the compound, the method comprises selecting the compound as a candidate promoter that promotes homologous recombination activity.

[0027] The method described in

[0025] , wherein the protein is a protein having the effect of reducing cell viability, and if the reduction in cell viability is suppressed in cells treated with the compound compared to cells not treated with the compound, the method comprises selecting the compound as a candidate inhibitor that inhibits homologous recombination activity, or if the reduction in viability of cells treated with the compound occurs more quickly than the reduction in viability of cells not treated with the compound, selecting the compound as a candidate promoter that promotes homologous recombination activity.

[0028] Introducing the nucleic acid construct according to any one of [1] to

[0012] into cancer cells of a cancer patient; and Measuring the expression of said protein. A method for diagnosing a homologous recombination deficient cancer, comprising:

[0029] The method described in

[0028] , wherein the cancer cells are cells isolated from the cancer patient, and introduction of the nucleic acid construct into the cancer cells is performed ex vivo.

[0030] The method described in

[0028] , wherein the protein is a protein whose expression within a cell can be detected as a signal, the introduction of a nucleic acid construct into the cancer cells is carried out by administering the nucleic acid construct to the cancer patient, and whether a signal of the protein can be detected from the cancer lesion is examined.

[0031] The method described in

[0028] , wherein the protein is a secreted protein whose expression within cells can be detected, introduction of a nucleic acid construct into the cancer cells is carried out by administering the nucleic acid construct to the cancer patient, and activity of the protein in blood isolated from the patient after administration of the nucleic acid construct is measured.

[0032] Introducing the nucleic acid construct according to any one of [1] to

[0012] into cancer cells of a cancer patient who is currently undergoing PARP inhibitor treatment or who has previously been diagnosed with a homologous recombination deficient cancer; and Measuring the expression of said protein. A method for detecting homologous recombination restored cancer cells, comprising:

[0033] The method described in

[0032] , wherein the cancer cells are cells isolated from the patient and introduction of the nucleic acid construct into the cancer cells is performed ex vivo.

[0034] The method described in

[0032] , wherein the protein is a protein whose expression within a cell can be detected as a signal, introduction of a nucleic acid construct into the cancer cells is carried out by administering the nucleic acid construct to the patient, and it is examined whether a signal of the protein can be detected from the cancer lesion.

[0035] Introducing the nucleic acid construct according to any one of [1] to

[0012] into cancer cells of a cancer patient; and Measuring the expression of said protein. The present invention relates to a method for predicting the efficacy of an anticancer drug against a homologous recombination-deficient cancer, the method comprising the steps of:

[0036] The method described in

[0035] , wherein the anticancer drug is a DNA damaging anticancer drug.

[0037] The method described in

[0036] , wherein the DNA damaging anticancer drug is a PARP inhibitor.

[0038] A method according to any one of claims 0035 to 0037, wherein the cancer cells are cells isolated from the cancer patient, and introduction of the nucleic acid construct into the cancer cells is carried out ex vivo.

[0039] A method according to any one of

[0035] to

[0037] , wherein the protein is a protein whose expression within a cell can be detected as a signal, introduction of a nucleic acid construct into the cancer cell is carried out by administering the nucleic acid construct to the cancer patient, and whether or not a signal of the protein is detected from a cancer lesion is examined.

[0040] A method according to any one of

[0035] to

[0037] , wherein the protein is a secreted protein whose expression in cells can be detected, introduction of a nucleic acid construct into the cancer cells is carried out by administering the nucleic acid construct to the cancer patient, and activity of the protein in blood isolated from the patient after administration of the nucleic acid construct is measured.

[0041] A method for treating homologous recombination restored cancer, comprising administering to a patient having homologous recombination restored cancer a nucleic acid construct according to any one of [1] to

[0012] , wherein the gene sequence is a gene sequence encoding a protein having an effect of reducing cell viability.

[0042] The method described in

[0041] , wherein the nucleic acid construct is administered locally within or near the patient's tumor.

[0043] The method described in

[0041] or

[0042] , wherein the homologous recombination restored cancer is a PARP inhibitor resistant cancer.

[0044] A method for predicting whether a genetic mutation identified in a cancer patient is a pathogenic mutation that impairs homologous recombination activity, comprising: constructing an expression vector that expresses a mutant gene having the same mutation as the mutation; preparing an expression vector that expresses a wild-type gene that does not have the mutation; introducing a mutant gene expression vector and a wild-type gene expression vector into the cell lacking the gene, and also introducing the nucleic acid construct according to any one of [1] to

[0012] ; Measuring the expression of the protein in a cell into which any of the expression vectors and nucleic acid constructs have been introduced; wherein, when the expression level of the protein in a cell into which a mutant gene expression vector has been introduced is lower than the expression level of the protein in a cell into which a wild-type gene expression vector has been introduced, it is indicated that the mutation is a pathogenic mutation that impairs homologous recombination activity. The present invention provides a novel means for evaluating the homologous recombination activity of cells not in a relative manner but in an absolute manner. The nucleic acid construct of the present invention is a split nucleic acid construct consisting of a set of a first nucleic acid molecule containing a mutant gene sequence and a second nucleic acid molecule containing a complementary region capable of reproducing a gene sequence capable of expressing an active protein by complementing the mutated portion of the mutant gene sequence by homologous recombination. With the nucleic acid construct of the present invention, the presence or absence of homologous recombination activity can be detected in a very short time in a transient expression system. The nucleic acid construct of the present invention is useful as a measurement reagent or measurement kit for homologous recombination activity, and can be used as a screening system for drugs that affect homologous recombination activity, a diagnostic agent for homologous recombination-deficient cancer, a detection agent for homologous recombination-recovered cancer cells, a companion diagnostic agent for predicting the effect of an anticancer drug on homologous recombination-deficient cancer, and the like. Furthermore, by using a gene sequence that encodes a protein that has the effect of reducing the survival rate of cells, such as a suicide gene, as the gene sequence, a nucleic acid construct that selectively exerts a cytocidal effect on cells having homologous recombination activity can be obtained. Anticancer drug-resistant cancers with restored homologous recombination activity, which is a major problem in PARP inhibitor treatment, currently have no treatment, but the nucleic acid construct of the present invention makes it possible to treat such resistant cancers. Furthermore, the construct of the present invention can be used to determine or predict whether a mutation identified in a cancer patient is a pathogenic mutation. FIG. 1 is a schematic diagram for explaining the structure of the integrated and split nucleic acid constructs carrying the Nluc gene, prepared in Example A. FIG. 2 is a structural diagram of pIRES used in the preparation of the plasmid vector-type nucleic acid construct in the examples (partially modified from the structural diagram of pIRES Vector Information from Clontech). FIG. 3 is a structural diagram of pUC19 used in the preparation of one molecule of the split nucleic acid construct in the examples (from the pUC19 DNA data sheet from Takara Bio). FIG. 4 is an example of the results of a luciferase assay using a nucleic acid construct prepared using the Nluc gene. The luciferase activity after introduction of the construct was measured over time in the wild-type and RAD54 / RAD54B double-deficient strains of Nalm-6 cells. FIG. 5 is an example of the results of a luciferase assay using a nucleic acid construct prepared using the Nluc gene. The luciferase activity 4 hours after introduction of each construct was compared between the wild-type and RAD54 / RAD54B double-deficient strains of Nalm-6 cells. FIG. 6 is an example of the results of a luciferase assay using a nucleic acid construct prepared using the Nluc gene. The luciferase activity 4 hours after introduction of each construct was compared between HT1080 cells and MDA-MB-436 cells. The results of investigating the effect of RAD51 gene knockdown on luciferase activity detected after construct introduction into Nalm-6 cells. Schematic diagram explaining the structure of integrated and split nucleic acid constructs carrying the suicide gene DT-A, prepared in Example B. The cell killing effect of the nucleic acid construct prepared using the DT-A gene. (A) The result of comparing the survival rate 96 hours after introduction of the construct between a wild type strain and a RAD54 / RAD54B double-deficient strain of Nalm-6 cells. (B) The result of comparing the survival rate 96 hours after introduction of the construct between a human tumor-derived cell line HT1080 that is not deficient in homologous recombination and a human tumor-derived cell line MDA-MB-436 that is deficient in homologous recombination activity due to BRCA1 deficiency. Schematic diagram explaining the structure of integrated and split nucleic acid constructs carrying the suicide gene HSV-TK, prepared in Example C. This shows the results of comparing the cell-killing effects of each construct prepared using the HSV-TK gene in MDA-MB-436 and HT1080 cells.Schematic diagrams illustrating the structures of integrated and split nucleic acid constructs carrying the suicide gene TK30 gene, prepared in Example D. The upper row shows the positive control TK30 expression construct, the middle row shows the integrated construct, and the lower row shows the split construct. A diagram illustrating the mutation introduction site of TK30. Cell-killing effect of nucleic acid constructs prepared using the TK30 gene. The survival rates 72 hours after introduction of the construct were compared between Nalm-6 cells and their HR-deficient cells (RAD54 / RAD54B double-deficient strain). MDA-MB-436 cells (BRCA1-deficient cells) and Olap. R 16 shows the results of comparing the survival rates after introduction of each construct prepared using the TK30 gene between Nalm-6 cells (MDA-MB-436 cells with restored homologous recombination activity) and Nalm-6 cells (MDA-MB-436 cells with restored homologous recombination activity). Schematic diagram for explaining the structure of the integrated and split nucleic acid constructs carrying the suicide gene CD::UPRT gene prepared in Example E. The upper row shows constructs expressing CD::UPRT fusion protein, CD, and UPRT, respectively, the middle row shows an integrated construct, and the lower row shows a split construct. In order to examine whether the cytotoxicity of 5-FC is enhanced by coexpression of CD and UPRT, the results of comparing the survival rates after introduction of each construct prepared using the CD::UPRT gene between Nalm-6 cells and its HR-deficient cells (RAD54 / RAD54B double-deficient strain). MDA-MB-436 cells and Olap R1 shows the results of comparing the survival rates of cells after introduction of each construct prepared using the CD::UPRT gene. Schematic diagrams illustrating the structures of the integrated and split nucleic acid constructs carrying the CeNL gene prepared in Example F. The upper row shows the positive control CeNL expression construct, the middle row shows the integrated construct, and the lower row shows the split construct. The results of measuring luciferase activity over time after introduction of a construct prepared using the CeNL gene in a wild-type strain and a RAD54 / RAD54B double-deficient strain (homologous recombination-deficient cell) of Nalm-6 cells. The results of introducing the construct of Example A using Nluc or the construct of Example F using CeNL into Nalm-6 cells and performing a luciferase assay after 2 hours. The results of introducing the construct of Example A using Nluc or the construct of Example F using CeNL into Nalm-6 cells and performing a luciferase assay after 4 hours. MDA-MB-436 cells (cells lacking homologous recombination activity due to BRCA1 deficiency) and Olap R The results are a comparison of luciferase activity after transfection of the construct using CeNL between MDA-MB-436 cells and Olap cells (MDA-MB-436 cells in which homologous recombination activity was restored). R The construct of Example A using Nluc or the construct of Example F using CeNL were transfected into MDA-MB-436 cells and Olap cells, and luciferase assay was performed 2 hours later. R The results are shown in Table 1. The cells were transfected with the vector and luciferase assay was performed 4 hours later. The nucleic acid construct of the present invention is a construct composed of a set of two nucleic acid molecules. In this specification, this nucleic acid construct may be referred to as a split-type nucleic acid construct. The first nucleic acid molecule comprises a promoter region and a mutant gene sequence having a cleavage site within a gene sequence encoding a protein, the mutant gene sequence being located downstream of the promoter region. The second nucleic acid molecule comprises a complementary region composed of a first homologous region and a second homologous region, capable of replacing a partial region including the cleavage site in the mutant gene sequence by homologous recombination (HR). The complementary region is a region consisting of a base sequence selected from the following (i) to (iii): (i) A continuous subsequence in a gene sequence encoding the protein, the subsequence including the upstream and downstream regions adjacent to the cleavage site in the mutant gene sequence. (ii) A nucleotide sequence that is homologous to a partial sequence of (i) and encodes the same amino acid sequence as said partial sequence. (iii) A contiguous partial sequence in a gene sequence encoding a protein having 80% or more sequence identity with the protein and the same activity as the protein, said partial sequence having homology to the partial sequence of (i). In the present invention, homologous recombination (HR) refers to a recombination reaction that occurs by RAD51-dependent HR (sometimes referred to as "standard HR"). In general, HR occurs when there is 100% homology between a homologous region and a substrate sequence (a target sequence for the recombination reaction), but it is widely known in the art that standard RAD51-dependent HR can occur even if some nucleotide differences are present. The gene sequence employed in the nucleic acid construct of the present invention is not particularly limited as long as it is a gene sequence that codes for a protein. Hereinafter, the protein encoded by this gene sequence may be referred to as protein A in the present specification. In the nucleic acid construct of the present invention, in a cell having HR activity, a partial region containing a cleavage site (when the mutant gene sequence contains a stop codon upstream of the cleavage site, the stop codon and the cleavage site) in the mutant gene sequence in the first nucleic acid molecule is replaced by a complementary region of the second nucleic acid molecule by HR, and the gene sequence encoding protein A is reproduced, thereby generating protein A. On the other hand, in a cell not having HR activity, a reaction in which the cleavage site of the mutant gene sequence (when the mutant gene sequence contains a stop codon, the stop codon and the cleavage site) is replaced by the complementary region does not occur, so that protein A is not expressed. Therefore, based on the expression amount of protein A, preferably the expression level of the activity of protein A, it is possible to detect whether a cell into which a nucleic acid construct has been introduced has HR activity, or to selectively kill cancer cells with restored HR activity by utilizing the activity of protein A. The gene sequence encoding protein A restored by homologous recombination may be a sequence including the full length of the coding region of a naturally occurring gene, or may be a sequence consisting of a part of the coding region or a modified sequence. For example, among naturally occurring gene sequences, a sequence encoding a region in which a small number of residues (e.g., about several residues) that are not important for the activity of the protein are deleted from either or both of the C-terminus and N-terminus, or only a region encoding a domain necessary for the activity of the protein (e.g., a sequence encoding a protein fragment from which a domain not necessary for the activity of the protein itself, such as a domain necessary for membrane localization, has been removed) may be used as the gene sequence in the present invention. In addition, genes with various modifications are widely used commercially for fluorescent proteins and luminescent proteins, and gene sequences encoding such non-natural proteins may be used. Furthermore, a gene sequence capable of expressing two or more proteins as a fusion protein or a gene sequence capable of expressing polycistronically may be used as a gene sequence encoding protein A. That is, the term protein A includes a fusion protein and a set of two or more proteins expressed polycistronically. Typical examples of such an embodiment include a gene sequence encoding protein α, whose activity can be measured directly or indirectly using a phenotypic change caused in cells by the activity of the protein as an index, and a protein β, which has the effect of enhancing the activity of the protein α, as a fusion protein, and a gene sequence capable of being expressed polycistronically. Even in such an embodiment, the sequence encoding each of the constituent proteins (for example, the above-mentioned proteins α and β) may be a sequence including the full length of the coding region of a naturally occurring gene, or may be a sequence consisting of a part of it or modified (specific examples are as described above). Techniques for expressing multiple proteins polycistronically are well known, and for example, multiple protein coding sequences may be linked via an IRES sequence or a 2A peptide coding sequence. Specific examples of IRES sequences and 2A peptide coding sequences include the IRES sequence shown in SEQ ID NO: 88 and the IRES2 sequence shown in SEQ ID NO: 89 (both of which are derived from the internal rebosome entry site of encephalomyocarditis virus (ECMV)), and 2A peptide coding sequences include the P2A coding sequence (SEQ ID NO: 90, 91), the E2A coding sequence (SEQ ID NO: 92, 93), the F2A coding sequence (SEQ ID NO: 94, 95), etc. As is well known, when an IRES sequence is used, it is necessary to add a stop codon to the upstream gene sequence, and when a 2A peptide coding sequence is used, a stop codon must not be added to the upstream gene. Protein A is preferably a protein whose expression level can be measured quickly and easily, and for example, it is preferable to use as protein A a protein whose activity can be measured directly or indirectly using as an indicator a phenotypic change caused in cells by the activity of the protein, rather than measuring the production or accumulation amount of the protein itself. Examples of such proteins include proteins whose expression in cells can be detected by signals such as luminescence due to reaction with a substrate or fluorescence of the protein itself. Another example is a protein that has the effect of reducing cell viability, in which case the activity of the protein can be indirectly measured as a reduction in cell viability. The term "measuring the expression (expression level) of protein A" includes measuring the production or accumulation level of protein A and measuring the activity of protein A. As described above, in the present invention, it is preferable to measure the expression of protein A by directly or indirectly measuring the activity of protein A, that is, to employ as protein A a protein whose activity can be directly or indirectly measured. Preferred examples of protein A include proteins that have the effect of reducing cell viability and proteins whose expression in cells can be detected. Specific examples of genes encoding proteins that have the effect of reducing cell viability include, but are not limited to, suicide genes, DNA damage-inducing genes, and DNA repair inhibitor genes. Suicide genes include genes that code for proteins that are themselves toxic or damaging to cells, genes that code for proteins that act on other compounds to produce toxic substances, and genes that code for natural or non-natural proteins that exert cytotoxicity by inhibiting endogenous proteins through a dominant-negative effect. Specific examples of suicide genes include the diphtheria toxin A fragment gene (DT-A) (DT-A protein itself is toxic to cells and kills them), herpes virus-derived thymidine kinase gene (HSV-TK) (HSV-TK protein acts on ganciclovir or 5-iodo-2'-fluoro-2'deoxy-1-beta-D-arabino-furanosyl-uracil (FIAU) to convert it into a toxic substance with DNA synthesis inhibitory activity; when this gene is used as a suicide gene, cells must be treated with ganciclovir or FIAU), p53 gene (overexpression of the gene induces cell growth cycle arrest or apoptosis, resulting in cell death), and cytosine deaminase (CD) gene (CD converts 5-fluorocytosine (5-FC) to 5-fluorouracil by deamination. 5-fluorouracil is metabolized in cells and converted into a substance that inhibits DNA synthesis and RNA synthesis, inducing cell death (Austin EA and Huber BE. Mol Pharmacol. 1993 Mar; 43(3): 380-387.) and variants in which the action of these genes is enhanced. Specific examples of modified suicide genes include the TK30 gene, which is a mutant of the HSV-TK gene (see Example D below), and the CD::UPRT gene, which is a fusion gene of the CD gene and the uracil phosphoribosyltransferase (UPRT) gene (see Example E below). The TK30 gene is a mutant of the HSV-TK gene that has been modified to enhance the action of ganciclovir by introducing mutations into amino acids near the active center of the HSV-TK gene (specifically, the 152nd alanine is replaced by valine, the 159th leucine by isoleucine, the 160th isoleucine by leucine, the 161st phenylalanine by alanine, the 168th alanine by tyrosine, and the 169th leucine by phenylalanine). The TK30 gene is known to exhibit stronger cytotoxicity than wild-type HSV-TK (Kokoris MS et al. Gene Ther. 1999, Aug;6(8):1415-1426.).The CD::UPRT gene is a fusion gene that combines UPRT, which is known to catalyze the metabolic pathway by which 5-fluorouracil is converted into a toxic substance within cells, with CD. Combining these genes enhances the cytotoxicity of 5-fluorocytosine and provides a bystander effect (Tiraby M et al. FEMS Microbiol Lett. 1998 Oct 1;167(1): 41-49., Bourbeau D et al. J Gene Med. 2004 Dec; 6(12): 1320-1332.). In Example E below, using budding yeast-derived CD (NP_015387, SEQ ID NO: 53) and UPRT (NP_011996, SEQ ID NO: 55), the full length of the base sequence (SEQ ID NO: 52) obtained by codon-optimizing the budding yeast CD gene (Fcy1 gene, NM_001184159, SEQ ID NO: 82) and the almost full length of the base sequence (SEQ ID NO: 54) obtained by codon-optimizing the budding yeast UPRT gene (Fur1 gene, NM_001179258, SEQ ID NO: 83) were linked by an alanine codon, and the full length of the budding yeast-derived CD and the 3rd residue to the C-terminal residue of the budding yeast-derived UPRT were linked by one alanine residue. In this example, a fusion gene encoding a fusion protein (SEQ ID NO: 51) having the above structure is used as the CD::UPRT gene, but the species from which both genes originate and the fusion method are not limited to this specific example, and the CD::UPRT gene can be freely designed by appropriately optimizing the codons of known gene sequences encoding CD and UPRT (as defined in the above gene sequence, the entire natural gene sequence may be a sequence consisting of a part of the natural gene sequence or a sequence with modifications) according to the animal species of the cell into which the construct is to be introduced, and linking the two together via one or more linker residues as desired. Also, as described above, it is possible to express CD and UPRT polycistronically.Examples of known CD genes and UPRT genes include the above-mentioned Fcy1 gene and Fur1 gene derived from budding yeast, as well as the codA gene, which is a CD gene of Escherichia coli (the protein ID of the CD encoded by this gene is NP_414871.1, SEQ ID NOs: 84 and 85), and the upp gene, which is a UPRT gene of Escherichia coli (the protein ID of the UPRT encoded by this gene is NP_416993.2, SEQ ID NOs: 86 and 87). Further specific examples of suicide genes include natural nucleases such as restriction enzymes and meganucleases, and artificial nucleases such as ZFN, TALEN, and CRISPR systems (these are also specific examples of DNA damage-inducing genes). However, suicide genes are not limited to the above specific examples. Examples of genes encoding proteins whose expression in cells can be detected include, but are not limited to, luminescent enzyme genes such as luciferase genes (including genes encoding secreted luciferase), fluorescent protein genes (including GFP, CFP, OFP, RFP, YFP, and modified versions thereof, which encode natural or artificial fluorescent proteins), cell surface antigen genes, secreted protein genes, and membrane protein genes. Luminescent enzyme genes and fluorescent protein genes are genes that encode proteins whose expression in cells can be detected as signals such as luminescence or fluorescence, and are examples of genes that can be preferably used in the present invention. In addition, as with suicide genes, modified forms of the above-mentioned various genes whose functions have been enhanced can also be used. A specific example is the CeNL (Cyan enhanced nano-lantern) gene (see Example F below), which encodes a fusion protein of the chemiluminescent protein Nluc and the cyan fluorescent protein mTurquoise2 (Goedhart J et al. Nat Commun. 2012, Mar 20;3: 751.). It is known that the CeNL gene enhances luminescence intensity through Forester resonance energy transfer (FRET) between Nluc and mTurquoise2 (Suzuki K et al. Nat Commun. 2016, Dec 14;7: 13718.). The promoter contained in the first nucleic acid molecule is not particularly limited, and may be any promoter that can exert promoter activity in cells (typically mammalian cells such as human cells). In general, a promoter that exerts a strong constitutive promoter activity in cells into which the nucleic acid construct of the present invention is introduced can be preferably used, but an inducible promoter that exerts promoter activity under certain conditions may also be used. In the following examples, a human cytomegalovirus promoter that exerts a strong constitutive promoter activity in cells is used, but is not limited thereto. It is not limited to promoters derived from viruses, and any sequence that has promoter activity in cells, preferably in human cells, may be used, regardless of origin. In addition, when a mutant gene sequence is functionally linked downstream of a promoter region, it means that the promoter region and the mutant gene sequence are arranged so that the gene sequence formed on the first nucleic acid molecule after the homologous recombination reaction can express protein A under the control of the promoter, and in the nucleic acid construct of the present invention, it is sufficient that the mutant gene sequence is arranged downstream of the promoter region on the first nucleic acid molecule. The mutant gene sequence contained in the first nucleic acid molecule is a base sequence having a structure in which a mutation (i.e., a cleavage site) is introduced into the gene sequence encoding protein A. When a gene sequence derived from a eukaryote is used as the gene sequence encoding protein A, the mutant gene sequence may be designed using the base sequence of an mRNA from which introns have been removed. The original gene sequence may be a naturally occurring gene sequence as long as it encodes protein A, or may be a sequence containing one or more bases (typically a silent mutation) different from the natural gene sequence at a site other than the mutation introduction site. A stop codon may be included upstream of the cleavage site, and in that case, the stop codon and the cleavage site are introduced into the gene sequence encoding protein A so as to be arranged in the order of [stop codon]-[cleavage site] from the 5' side. It is not essential to include a stop codon upstream of the cleavage site, but when the first nucleic acid molecule is prepared as a circular molecule and used in its circular form (introduced into a cell or administered to a patient) without being cleaved at the cleavage site to linearize it, it is desirable to also include a stop codon upstream of the cleavage site in order to reliably stop the expression of active protein A from the mutant gene sequence in the first nucleic acid molecule in cells lacking homologous recombination activity. The stop codon and the cleavage site may be directly linked, or some base sequence may exist between them, but it is preferable that the chain length of [stop codon]-[cleavage site] is approximately 50 bases or less, for example, approximately 30 bases or less. A typical example of the cleavage site is a restriction enzyme recognition site, but is not limited thereto. In addition to cleavage by a restriction enzyme, for example, cleavage can be performed using a genome editing technique that causes DNA strand cleavage. A specific example is cleavage by the CRISPR / Cas system, that is, cleavage by a complex of a guide RNA and a Cas protein such as Cas9. When using the complex, a PAM sequence is introduced at an appropriate position so that cleavage by the complex occurs at an appropriate position downstream of the stop codon. In the case of Cas9 derived from Streptococcus pyogenes type II, which is the most commonly used known CRISPR / Cas system, the PAM sequence is 5'-NGG (N is A, T, G, or C). The complex cleaves the nucleic acid construct several bases upstream of the PAM sequence. In the case of cleavage by the CRISPR / Cas system, the PAM sequence and the actual cleavage site several bases upstream thereof constitute the cleavage site in the present invention. In the present invention, the unit of nucleic acid chain length is expressed as "base", but when the nucleic acid is a double-stranded nucleic acid, it means "base pair". Furthermore, the [cleavage site] or [stop codon]-[cleavage site] may be inserted into the gene sequence encoding protein A (in this case, the first nucleic acid molecule includes the entire gene sequence encoding protein A), or a part of the gene sequence encoding protein A may be replaced with the [cleavage site] or [stop codon]-[cleavage site] (in this case, the first nucleic acid molecule includes a sequence lacking a part of the gene sequence encoding protein A). In the present invention, the "homology" of a base sequence has the same meaning as the "sequence identity" of a base sequence, and is a percentage obtained by aligning two base sequences to be compared so that as many bases as possible match, and dividing the number of matched bases by the total number of bases. When aligning, gaps are inserted appropriately in one or both of the two sequences to be compared, as necessary. Such alignment of sequences can be performed using well-known programs such as BLAST, FASTA, and CLUSTAL W. When gaps are inserted, the total number of bases is the number of bases counted with one gap as one base. When the total number of bases counted in this way differs between the two sequences to be compared, the homology (%) is calculated by dividing the number of matched bases by the total number of bases of the longer sequence. The "sequence identity" of amino acid sequences is calculated in the same manner. Hereinafter, in the mutant gene sequence, the upstream side of the [cleavage site] (when a stop codon is also included, [stop codon]-[cleavage site]) is referred to as the 5' region, and the downstream side is referred to as the 3' region. When used in a linearized state after cleavage at the cleavage site, a protein A fragment without a region downstream of the cleavage site is generated in cells where HR does not occur. In addition, when the circular construct is used as is, translation stops due to the introduced stop codon in cells where HR does not occur, and a protein A fragment consisting of the 5' region is generated. Therefore, the position of the [cleavage site] or [stop codon]-[cleavage site] in the mutant gene sequence is determined while taking into consideration the size of the 5' region so that the 5' region fragment does not exhibit the activity of protein A. The size of the 5' region may be appropriately set according to the type of protein A, and is not particularly limited, but generally, the size of the 5' region can be about 60 / 100 or less of the full length of protein A, for example, 50 / 100 or less, 40 / 100 or less, 30 / 100 or less, or 25 / 100 or less. When protein A is a fusion protein of protein α having an effect of reducing cell viability or emitting a signal, and protein β having an effect of enhancing the effect of protein α, and protein β is fused to the C-terminus of protein α, the size of the 5'-side region can be about 60 / 100 or less of the protein α portion, for example, 50 / 100 or less, 40 / 100 or less, 30 / 100 or less, or 25 / 100 or less. Alternatively, a cleavage site can be located upstream of the base sequence encoding the amino acid that serves as the active center so that the 5'-side region is a region N-terminal to the active center of protein A (when protein A is a fusion protein in which protein β is fused to the C-terminus of protein α, the active center of protein α) to be the 5'-side region, thereby preventing the protein A fragment consisting of the 5'-side region from exerting its activity. The lower limit of the size of the 5'-side region is not particularly limited, but is usually set to a size of 50 bases or more. The complementary region contained in the second nucleic acid molecule is a partial region in a mutant gene sequence that can replace the partial region containing the cleavage site by homologous recombination, and is composed of a first homologous region and a second homologous region, and is composed of a base sequence selected from the following (i) to (iii): (i) A continuous subsequence in the gene sequence encoding protein A, which subsequence includes the upstream and downstream regions adjacent to the cleavage site in the mutant gene sequence. (ii) A nucleotide sequence that is homologous to a partial sequence of (i) and encodes the same amino acid sequence as the partial sequence of (i). (iii) A contiguous partial sequence in a gene sequence encoding a protein having 80% or more sequence identity with protein A and the same activity as protein A, said partial sequence having homology with the partial sequence of (i). The entire 5'-side region in the mutant gene sequence contained in the first nucleic acid molecule, or a part of the 3'-end thereof (i.e., at least a part of the 3'-end of the 5'-side region) corresponds to the upstream region adjacent to the cleavage site (or the stop codon and the cleavage site) in the mutant gene sequence in the sequence (i) above (hereinafter, for convenience, referred to as the "adjacent upstream region"). In the sequence (i), this adjacent upstream region constitutes the first homologous region. At least a part of the 3'-end of the 5'-side region in the mutant gene sequence and the adjacent upstream region in (i) above are conveniently referred to as the "first homologous region set". In addition, the entire 3'-side region or a part of the 5'-end thereof (i.e., at least a part of the 5'-end of the 3'-side region) corresponds to the downstream region adjacent to the cleavage site (or the stop codon and the cleavage site) in the mutant gene sequence in the sequence (i) above (the "adjacent downstream region"). In the sequence (i), this adjacent downstream region constitutes the second homologous region. At least a part of the 5' end of the 3' region in the mutant gene sequence and the adjacent downstream region in (i) above are conveniently referred to as the "second homologous region set." The length of each homologous region may be at least 20 bases, and may be, for example, 50 bases or more, 60 bases or more, 70 bases or more, or 80 bases or more. The upper limit of the length is not particularly limited, but is usually 10,000 bases or less, and may be, for example, 5,000 bases or less, 1,000 bases or less, 500 bases or less, or 300 bases or less. The embodiment in which the complementary region is the base sequence (i) is an embodiment in which the homology between the first set of homologous regions and the homology between the second set of homologous regions are 100%, and the corresponding sequences are completely identical. In the embodiment where the complementary region is the base sequence of (ii), the encoded amino acid sequence is identical to (a partial sequence of) the amino acid sequence of protein A, but at least one of the homology between the first set of homologous regions and the homology between the second set of homologous regions is less than 100%. It is widely known in the art that RAD51-dependent HR occurs when the corresponding sequences do not completely match but have a certain level of homology. The sequence of (ii) preferably has a homology of 90% or more with the partial sequence of (i), more preferably 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more. For example, the homology between the first set of homologous regions is preferably 90% or more, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more, and the homology between the second set of homologous regions is preferably 90% or more, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more. In the sequence of (ii), it goes without saying that the regions corresponding to the adjacent upstream region and the adjacent downstream region in (i), respectively, constitute the first homologous region and the second homologous region. The base sequence (iii) is a contiguous partial sequence in a gene sequence encoding a protein having 80% or more, for example 85% or more, 90% or more, 95% or more, or 98% or more sequence identity with protein A and the same activity as protein A, and is a base sequence having homology to the partial sequence of (i). This is an embodiment in which at least one of the homology between the first set of homologous regions and the homology between the second set of homologous regions is less than 100%, and the encoded amino acid sequence does not match the amino acid sequence of protein A. Even if the encoded amino acid sequence is partially different from the amino acid sequence of protein A, it can be adopted as the sequence of the complementary region as long as it has the activity of protein A. As (iii), it is preferable that the homology with the partial sequence of (i) is 90% or more, for example, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more, and that the amino acid sequence that it encodes has a sequence identity of 80% or more, for example, 85% or more, 90% or more, 95% or more, or 98% or more, to the amino acid sequence encoded by the partial sequence of (i). Conservative substitution, i.e., substitution with an amino acid with similar chemical properties, is highly likely not to impair the properties and activity of the protein. Amino acids with similar side chains have similar chemical properties. Amino acids can be grouped according to the similarity of their side chains into groups of amino acids with aliphatic side chains (glycine, alanine, valine, leucine, isoleucine), groups of amino acids with aliphatic hydroxyl side chains (serine, threonine), groups of amino acids with amide-containing side chains (asparagine, glutamine), groups of amino acids with aromatic side chains (phenylalanine, tyrosine, tryptophan), groups of amino acids with basic side chains (arginine, lysine, histidine), groups of amino acids with acidic side chains (aspartic acid, glutamic acid), groups of amino acids with sulfur-containing side chains (cysteine, methionine), and the like. Substitution with another amino acid belonging to the same group is a conservative substitution. A typical example of the amino acid sequence encoded by the base sequence of (iii) is an amino acid sequence in which conservative substitutions have been introduced into the amino acid sequence encoded by the partial sequence of (i), but is not limited to this. The mutant gene sequence and the complementary region can be prepared by amplifying by PCR using appropriate primers from a known plasmid in which a gene encoding protein A has been incorporated, or from cultured cells or a cDNA library of a species expressing protein A. A stop codon is required at the 3' end of the mutant gene sequence. The complementary region is a sequence that serves as a substrate when replacing a part of the mutant gene sequence by homologous recombination, and is a region that is not translated before homologous recombination occurs, so there is no need to add a stop codon to the 3' end of the complementary region. The following examples describe examples in which a stop codon is introduced at the 3' end of the iNluc fragment and the iDTA fragment that correspond to the complementary region, and although it is acceptable to introduce a stop codon as in these examples, it is essentially unnecessary. During PCR amplification, in addition to a stop codon, an appropriate adapter sequence or the like used when cloning the nucleic acid construct of the present invention may be introduced into the two gene fragments, if desired. The sequence of the mutant gene sequence containing one or more bases different from the natural gene sequence at a site other than the mutation introduction site, and the sequences of (ii) and (iii) of the complementary region can be prepared by a method such as in vitro mutagenesis or artificial gene synthesis. The poly A addition signal may be arranged downstream of the mutant gene sequence in the first nucleic acid molecule (functionally linked downstream of the mutant gene sequence). The term "functionally linked" as used herein means that the poly A addition signal is arranged downstream of the mutant gene sequence so that an mRNA having a poly A tail is expressed from the gene sequence encoding protein A formed after the homologous recombination reaction. However, the poly A addition signal is not an essential element in the nucleic acid construct of the present invention. There are known mRNAs that function properly without poly A, such as histone mRNA, and it is possible to omit the poly A addition signal in the nucleic acid construct of the present invention. The complementary region may also be configured to cause replacement by homologous recombination, including a partial region adjacent to the mutant gene sequence in the first nucleic acid molecule. For example, the first homologous region contained in the complementary region may also include a region that is homologous to a partial region adjacent to the 5' end of the mutant gene sequence in the first nucleic acid molecule.Alternatively, the second homologous region contained in the complementary region may also include a region homologous to a partial region adjacent to the 3' end of the mutant gene sequence in the first nucleic acid molecule. Both the first and second nucleic acid molecules may be circular or linear molecules, and may be in the form of a nucleic acid fragment not incorporated in a vector, or may be incorporated in a plasmid vector or a viral vector. As described below, the nucleic acid construct of the present invention can be used as a medicine such as a therapeutic agent or a diagnostic agent, or as a reagent such as a measurement reagent. When the first nucleic acid molecule is a circular molecule, it may be used as a medicine or a reagent in the form of a circular molecule, or may be used in the form of a linear molecule obtained by cleaving the circular first nucleic acid molecule at a cleavage site inside the mutant gene sequence. In addition, the circular first nucleic acid molecule may be cleaved at a cleavage site inside the mutant gene sequence and at any site downstream of the mutant gene sequence (downstream of the polyA addition signal if a polyA addition signal is included) and upstream of the promoter region, and used as two nucleic acid molecules. When the second nucleic acid molecule is a circular molecule, it may be used as a medicine or reagent in the circular form, or may be cut outside the complementary region to form a linear form, but the second nucleic acid molecule is more likely to undergo a homologous recombination reaction when used in the circular form. When the first nucleic acid molecule and the second nucleic acid molecule are in a form incorporated into a vector, general elements such as a replication origin, a translation origin, and a selection marker gene for drug resistance or the like may be included in the vector sequence portion depending on the type of vector. The second nucleic acid molecule may be a nucleic acid molecule consisting of only the above-mentioned complementary region, but more preferably has one or more additional sequences linked to the complementary region, which can promote homologous recombination with the first nucleic acid molecule. When the second nucleic acid molecule is a circular molecule, it is preferable that it further comprises an additional sequence of 1 to 20,000 bases linked to the complementary region, and the additional sequence may be, for example, 20 bases or more, 50 bases or more, 100 bases or more, 200 bases or more, 500 bases or more, 700 bases or more, or 1,000 bases or more, or may be 10,000 bases or less, or 5,000 bases or less. When the second nucleic acid molecule is a linear molecule, it is preferable that an additional sequence of 1 to 10,000 bases is linked to at least one of the 5'-end and 3'-end of the complementary region. "An additional sequence of 1 to 10,000 bases is linked to at least one" means that when additional sequences are linked to both ends of the complementary region, each additional sequence has a size of 1 to 10,000 bases. In this case, the additional sequence may be, for example, 20 bases or more, 50 bases or more, 100 bases or more, 200 bases or more, 500 bases or more, 700 bases or more, or 1000 bases or more, or may be 8000 bases or less, or 5000 bases or less. The additional sequence must have a base sequence that is not homologous to the mutant gene sequence on the first nucleic acid molecule so that homologous recombination does not occur between the additional sequence and the mutant gene sequence on the first nucleic acid molecule. Not having homology means that there is no partial region in the additional sequence that is 20 bases or more in size and has 90% to 100% homology with a part or all of the mutant gene sequence. The additional sequence should not have any homology to any region outside the mutant gene sequence on the first nucleic acid molecule. Furthermore, when the nucleic acid construct of the present invention is introduced into a cell for use, it is desirable to adopt a base sequence that is not homologous to the genomic sequence of the cell to be introduced. For example, artificial nucleic acid molecules such as known plasmid expression vectors that are commercially available for transfection of prokaryotic cells such as E. coli, yeast, and eukaryotic cells such as mammalian cell lines, and that have not been used in the preparation of the first nucleic acid molecule, can be preferably used as the additional sequence. Furthermore, when the second nucleic acid molecule is a circular molecule containing an additional sequence, the additional sequence may have an appropriate cleavage site (restriction enzyme recognition site, etc.) that cleaves only the additional sequence and does not cleave the complementary region. When a cleavage site is provided in the additional sequence, the cleavage site may be set at a site approximately equidistant from both ends of the complementary region, and the second nucleic acid molecule after cleavage may be designed to have additional sequence fragments of similar size at both ends of the complementary region. The first and second nucleic acid molecules constituting the nucleic acid construct of the present invention may be DNA or RNA. One of the first and second nucleic acid molecules may be DNA and the other may be RNA, or both may be DNA, or both may be RNA. In general, DNA is preferred in the case of a plasmid vector type nucleic acid molecule. A typical example of a viral vector type nucleic acid molecule is a nucleic acid molecule that is a double-stranded DNA in the form of a viral expression plasmid, but viruses expressed from a viral expression plasmid are also included in the viral vector type nucleic acid molecule. The nucleic acid molecule that is a virus expressed from a viral expression plasmid is RNA if it is an RNA viral vector, and is DNA if it is a DNA viral vector. In the case of a nucleic acid molecule in a form that is not incorporated into a vector, it is usually DNA. In addition, the first and second nucleic acid molecules constituting the nucleic acid construct of the present invention may partially contain a nucleotide analog. Examples of nucleotide analogs include, but are not limited to, crosslinked nucleic acids such as LNA, ENA, and PNA, and modified bases such as Super T (registered trademark) and Super G (registered trademark). The nucleic acid construct of the present invention can be used for various applications, as described below. When the nucleic acid construct of the present invention is used as a reagent such as a measurement reagent, a diagnostic agent used in vitro, a companion diagnostic agent, and a detection agent, the first nucleic acid molecule and the second nucleic acid molecule may be provided in a form in which they are contained in separate containers, or the first nucleic acid molecule and the second nucleic acid molecule are mixed in advance at an appropriate ratio (for example, a molar ratio of about 1:0.001 to 1000, about 1:0.01 to 100, or about 1:0.1 to 10, and may also be about 1:0.2 to 5, or about 1:0.5 to 2) and provided in the same container. In addition, the nucleic acid construct of the present invention may be a liquid reagent or agent containing an appropriate additive for the stability of the first and second nucleic acid molecules, or may be a powdered reagent or agent obtained by freeze-drying the first and second nucleic acid molecules. When the nucleic acid construct of the present invention is used as a diagnostic agent or a therapeutic agent to be administered to a patient, such a diagnostic agent or therapeutic agent may be in a form in which the first nucleic acid molecule and the second nucleic acid molecule are formulated as separate agents, or in a form in which the first nucleic acid molecule and the second nucleic acid molecule are formulated as a single agent containing both the first nucleic acid molecule and the second nucleic acid molecule. A diagnostic agent or therapeutic agent containing the nucleic acid construct of the present invention includes an embodiment that includes a set of an agent containing a first nucleic acid molecule and an agent containing a second nucleic acid molecule, and an embodiment that is an agent containing a mixture of the first nucleic acid molecule and the second nucleic acid molecule. In the present invention, "introducing a nucleic acid construct (into a cell)" means co-introducing a first nucleic acid molecule and a second nucleic acid molecule. When the first nucleic acid molecule is a circular molecule, it can be introduced as it is, but it is preferable to cut it at a cutting site in the mutant gene sequence and linearize it before use. When the second nucleic acid molecule is a circular molecule, it may be cut at any site outside the complementary region (e.g., a cutting site in the additional sequence) and linearized before introduction into a cell, or it may be introduced into a cell as it is in a circular form. "Administering a nucleic acid construct to a patient" refers to administering a first nucleic acid molecule and a second nucleic acid molecule to a patient. The administration of the first nucleic acid molecule and the second nucleic acid molecule is preferably performed simultaneously or sequentially. When administered sequentially, either nucleic acid molecule may be administered first. The nucleic acid construct of the present invention can be used, for example, as a reagent for measuring homologous recombination activity. In this embodiment of use, a gene sequence encoding a protein whose expression in cells can be detected can be preferably used as the gene sequence encoding protein A. The nucleic acid construct of the present invention is introduced (meaning co-introduction of the first and second nucleic acid molecules as defined above) into test cells in which homologous recombination activity is to be measured and into cells having normal homologous recombination activity or cells not deficient in homologous recombination activity (HR non-deficient control cells), and the expression of protein A is measured, and the expression amount of protein A (protein A signal) between the test cells and the HR non-deficient control cells is compared. The introduction of the nucleic acid molecule may be transient. In the present invention, "introducing a nucleic acid construct" means co-introduction of the first and second nucleic acid molecules. Furthermore, cells deficient in homologous recombination activity may be used as HR deficient control cells. The HR non-deficient control cell is not particularly limited as long as it has normal homologous recombination activity. HR-related genes (ATM, CDK1, CDK12, MRE11, NBS1, RAD50, PLK1, RNF1, RNF8, RNF168, ARID1A, SMARCAD1, CHD1, CHD4, LEDGF, RBBP8 (CtIP), BLM, Exo1, DNA2, BRCA1, BRCA2, PALB2, BARD1, BRIP1, BAP1, RAD51B, RAD51C, RAD51D, RAD51AP1, RAD52, XRCC As the HR-deficient control cell, any of many known wild-type mammalian cell lines that do not have a mutation in any of the following genes (e.g., HR-deficient GFP-1 ... Specific examples of known mammalian cell lines having normal homologous recombination activity include, for example, human cell lines, Nalm-6 (human pre-B cell leukemia cell-derived cell line) and HT1080 (human fibrosarcoma cell line), which are also used in the Examples below, U2OS (human osteosarcoma-derived cell line), HeLa (human cervical carcinoma-derived cell line), HCT116 (human colon adenocarcinoma-derived cell line), MCF-7 (human breast adenocarcinoma-derived cell line), HAP1 (human chronic myeloid leukemia-derived cell line), HEK293 (human fetal kidney-derived cell line), TIG-7 (human lung-derived cell line), TIG-3 (human lung-derived cell line), iPS cells (human induced pluripotent stem cells: established from normal human cells), and ES cells (human embryonic stem cells), but are not limited to these. Cell lines derived from human hereditary breast cancer and hereditary ovarian cancer have mutations in the HR-related genes BRCA1 and BRCA2, and many of them are deficient in homologous recombination activity, so they are not suitable as HR-non-deficient control cells.Whether or not a given cell line has normal homologous recombination activity (whether or not it is not deficient in homologous recombination activity) can be examined using the nucleic acid construct of the present invention, or by using a conventionally known technique. When examined using the nucleic acid construct of the present invention, if the cell line has homologous recombination activity at the same level or higher than that of the specific example of the HR non-deficient control cell described above, it may be judged to be a cell having normal homologous recombination activity (not deficient in homologous recombination activity). As an additional positive control, a construct (positive control construct) in which a normal gene sequence encoding protein A is linked under the control of a promoter may be introduced into the test cells (and, if desired, the control cells). The lower the homologous recombination activity of the test cells, the lower the expression of protein A in the test cells (the protein A signal detected from the test cells). Thus, a lower protein A signal from the test cells compared to the protein A signal from the HR-non-deficient control cells is an indication that the test cells have a lower homologous recombination activity. As the HR-deficient control cell, for example, a cell line derived from a homologous recombination-deficient cancer can be used. Cell lines derived from a homologous recombination-deficient cancer having a mutation in the above-mentioned HR-related gene (for example, as an example of a human cell line, a cell line derived from human hereditary breast cancer and hereditary ovarian cancer having a mutation in the BRCA1 gene or the BRCA2 gene) and cell lines produced by introducing mutations into one or more HR-related genes or methods for producing the same are known, and any of such cell lines can be used as the HR-deficient control cell. Since the technology of gene knockout has also been established, HR-deficient cells of various animal species can be prepared by knocking out one or more HR-related genes in cultured mammalian cells having normal homologous recombination activity.Specific examples of known homologous recombination-deficient cell lines include human cell lines such as HR-deficient human cell lines derived from Nalm-6 cells (RAD54-deficient, RAD54B-deficient, and RAD54 / RAD54B double-deficient cell lines) used in the following Examples, as well as cell lines from the American Type Culture Collection (ATCC). HR-deficient human cell lines available from the Horizon Cell Collection include HCC1395 (human breast cancer cell line, BRCA1-deficient), HCC1599 (human breast cancer cell line, BRCA2-deficient), HCC1937 (human breast cancer cell line, BRCA1-deficient), MDA-MB-436 (human breast cancer cell line, BRCA1-deficient), DoTc2-4510 (human cervical cancer cell line, BRCA2-deficient), RL95-2 (human uterine cancer cell line, BRCA2-deficient), AGS (human gastric cancer cell line, CHD1-deficient), SNU-5 (human gastric cancer cell line, CHD1-deficient), UWB1.289 (human ovarian cancer cell line, BRCA1-deficient), CAPAN-1 (human pancreatic cancer cell line, BRCA2-deficient), and LNCaP (human prostate cancer cell line, BRCA2-deficient); and Examples of HR-deficient human cell lines available from Discovery include genetically modified HAP1 cell lines (human chronic myeloid leukemia-derived cell lines) (ATRX-deficient, CHD1-deficient, CHD4-deficient, EME1-deficient, EME2-deficient, FANCA-deficient, FANCC-deficient, FANCE-deficient, FANCG-deficient, FANCF-deficient, LEDGF-deficient, LIG3-deficient, MUS81-deficient, RAD51AP1-deficient, RAD54L-deficient, and RNF8-deficient), genetically modified HCT116 cells (human colon adenocarcinoma-derived cell line) (ARID1A-deficient, LIG3-deficient, and PTEN-deficient), and genetically modified DLD-1 cell lines (human colon adenocarcinoma-derived cell line) (BRCA2-deficient).Specific examples of non-human animal cell lines include HR-deficient mice (frozen embryos) available from The NCI Mouse Repository (BRCA1-deficient mice, BRCA2-deficient mice); HR-deficient canine cells Parks (MCM8-deficient) (Sunetra Das et al., Mol Cancer Ther. Author manuscript; available in PMC 2020 Feb 1. Published in final edited form as: Mol Cancer Ther. 2019 Aug; 18(8): 1460-1471. Published online 2019 Jun 7. doi: 10.1158 / 1535-7163.MCT-18-1346). However, as described above, HR-deficient cells can be prepared in various animal species by creating cells in which one or more HR-related genes are knocked out, so the examples are not limited to these specific examples. In addition, the measurement reagent of the present invention can also be provided as a kit for measuring homologous recombination activity in combination with at least one item selected from a positive control plasmid (e.g., a plasmid in which a normal protein A coding sequence is placed downstream of a promoter and which is capable of constitutively expressing protein A), a negative control plasmid (a plasmid in which a gene sequence encoding protein A is not incorporated), HR non-deficient control cells, HR-deficient control cells, an instruction manual, and, in the case of protein A being an enzyme, an appropriate substrate substance. The nucleic acid construct of the present invention can also be used as a screening system for drugs that affect homologous recombination activity, for example. That is, the nucleic acid construct of the present invention can be used to identify candidate drugs that affect homologous recombination activity. The method for identifying candidate drugs that affect homologous recombination activity using the nucleic acid construct of the present invention includes introducing the nucleic acid construct of the present invention into a cell having normal homologous recombination activity, and then treating the cell with each compound of a compound group, or treating a cell having normal homologous recombination activity with each compound of a compound group, and then introducing the nucleic acid construct of the present invention into the cell; and measuring the expression of protein A in the compound-treated nucleic acid construct-introduced cell and the compound-untreated nucleic acid construct-introduced cell. As described above, the measurement of the expression of protein A is preferably the measurement of the activity of protein A. In this embodiment of use, the gene sequence encoding protein A is preferably a gene sequence encoding a protein whose expression in a cell can be detected, particularly a gene sequence encoding a protein whose expression in a cell can be detected as a signal, but a gene sequence encoding a protein that has an effect of reducing the viability of a cell can also be used. The first nucleic acid molecule is optionally cleaved at a cleavage site provided within the mutant gene sequence to make it linear, and then introduced together with the second nucleic acid molecule into cells with normal homologous recombination activity (HR normal cells, usually mammalian cells such as cultured human cells, are preferably used. Details are the same as for the above-mentioned HR non-deficient control cells), and then the cells are treated with individual compounds from a compound group such as a compound library (the order of introduction of the nucleic acid construct and compound treatment may be reversed), and the expression of protein A is measured to determine whether or not the compound treatment has caused a decrease or increase in the expression amount of protein A, preferably the activity level of protein A (whether or not a decrease or increase in homologous recombination activity has occurred). When protein A is luciferase, a change in luciferase activity (luminescence reaction) may be measured, and when protein A is a protein that has the effect of reducing cell viability, a change in cell viability may be examined.The decrease or increase in the expression of protein A can be evaluated by introducing a nucleic acid construct into cells having normal homologous recombination activity, measuring the expression of protein A without compound treatment, and comparing the measured value to determine whether the expression level is decreased or increased. In addition to HR normal cells, HR deficient cells lacking homologous recombination activity (e.g., RAD54 / RAD54B double deficient strains used in the Examples below; details are the same as for the HR deficient control cells above) may also be used. When protein A is a protein whose intracellular expression can be used as a signal to be detected, the compound can be selected as follows. When a lower signal for protein A is detected in compound-treated HR normal cells than in compound-untreated HR normal cells, the compound can be selected as a candidate HR inhibitor that inhibits HR activity. If the protein A signal rises earlier or a higher signal is detected in HR normal cells treated with a compound compared to HR normal cells not treated with the compound, the compound can be selected as a candidate HR promoter that promotes HR activity. When protein A is a protein that has the effect of reducing cell viability, the compound can be selected as follows. If the decrease in cell viability is suppressed in HR normal cells treated with a compound compared to HR normal cells not treated with the compound (i.e., if the viability of HR normal cells treated with a compound is higher than the viability of HR viable cells not treated with the compound), the compound can be selected as a candidate HR inhibitor that inhibits HR activity. If the decrease in viability of compound-treated HR normal cells occurs more quickly than the decrease in viability of compound-untreated HR cells, or if the viability of compound-treated HR normal cells is lower than the viability of compound-untreated HR cells, the compound can be selected as a candidate HR promoter that promotes HR activity. The nucleic acid construct of the present invention can also be used, for example, as a diagnostic agent for homologous recombination-deficient cancer. Deficiencies in homologous recombination have been reported in various cancers. To give specific examples, deficiencies in homologous recombination have been reported in breast cancer and ovarian cancer (including hereditary breast and ovarian cancer syndrome), as well as in colorectal cancer, endometrial cancer, gastric cancer, esophageal cancer, pancreatic cancer, hepatobiliary cancer, prostate cancer, liver cancer, non-small cell lung cancer, and small cell lung cancer (Non-Patent Documents 6 to 9), and it is expected that homologous recombination-deficient cancers will be discovered in various other cancers (including cancers of various animal species, including non-human animals) in the future. It has also been reported that molecular targeted cancer treatments such as EGFR inhibitors and BRAF inhibitors reduce homologous recombination activity (Science 20 Dec 2019: Vol. 366, Issue 6472, pp. 1473-1480, DOI: 10.1126 / science.aav4474). Homologous recombination deficient cancers in the present invention include various cancers, including the specific examples mentioned above. When used as a diagnostic agent for homologous recombination-deficient cancer, the nucleic acid construct of the present invention is introduced into cancer cells of a cancer patient, and the expression of protein A is measured. As described above, the measurement of the expression of protein A is preferably the measurement of the activity of protein A. As the gene sequence encoding protein A, a gene sequence encoding a protein whose expression in cells can be detected, particularly a protein whose expression in cells can be detected as a signal, can be preferably used, but a gene sequence encoding a protein having an effect of reducing cell viability can also be used. When homologous recombination is not deficient in a cell, homologous recombination occurs between a part of the mutant gene sequence in the first nucleic acid molecule and a complementary region in the second nucleic acid molecule, and a gene sequence capable of expressing protein A is reproduced in the cell, so that protein A is expressed. When protein A is a protein whose expression in cells can be detected as a signal, the signal of protein A is detected, and when protein A is a protein having an effect of reducing cell viability, the cell viability is reduced. On the other hand, when homologous recombination is deficient in a cell, a gene sequence capable of expressing protein A is not reproduced, so that protein A is not expressed. When protein A is a protein whose expression in cells can be detected as a signal, the signal of protein A is not detected, and when protein A is a protein that has the effect of reducing cell viability, the reduction in cell viability does not occur. In one embodiment of the diagnostic agent for homologous recombination-deficient cancer, the cancer cells are cells isolated from a cancer patient, and the introduction of a nucleic acid construct into the cancer cells is carried out ex vivo. Among the usage modes described below, (1) and (3) are specific examples of this embodiment. In this embodiment, as protein A, either a protein whose expression in cells can be detected or a protein that has the effect of reducing the viability of the cells can be adopted, but the former is more preferable. In this embodiment, the cancer patients include solid cancer patients and blood cancer patients. Cells of cancer tissue isolated from a cancer patient by biopsy or surgery, circulating tumor cells (CTCs) present in the blood of a cancer patient, and in the case of leukemia, cancer cells (leukemia cells) in the blood collected from a cancer patient can be used. If desired, the cancer cells may be concentrated before the introduction of the nucleic acid construct. The introduction of the nucleic acid construct may be transient. Whether or not the cancer cells are deficient in homologous recombination activity can be examined by measuring the homologous recombination activity of the cancer cells in the same manner as in the method of using the above-mentioned reagent for measuring homologous recombination activity. As the HR non-deficient control cells, in addition to the above-mentioned cells having normal homologous recombination activity, for example, non-cancer cells (non-cancer peripheral blood cells, etc.) collected from the same cancer patient can be used. Also, as the HR deficient control cells, the above-mentioned homologous recombination deficient cells can be used. In this embodiment, it is preferred to introduce the nucleic acid construct into cancer cells isolated from a cancer patient and into control cells (HR non-deficient control cells or HR-deficient control cells, or both), and compare the expression of protein A between the cancer cells and the control cells. When the expression / activity of protein A is detected in cancer cells, for example, when the expression level / activity level of protein A detected in cancer cells is higher than the expression level / activity level of protein A in HR-deficient control cells, it is indicated that the cancer of the cancer patient is not a homologous recombination-deficient cancer. When protein A is a protein whose expression in cells can be detected as a signal, when the signal of protein A is detected in cancer cells, for example, when the signal of protein A detected in cancer cells is higher than the signal of protein A in HR-deficient control cells, it is indicated that the cancer of the cancer patient is not a homologous recombination-deficient cancer. When protein A is a protein that has an effect of reducing cell viability, when the viability of cancer cells is reduced, for example, when the viability of cancer cells is lower than the viability of HR-deficient control cells, it is indicated that the cancer of the cancer patient is not a homologous recombination-deficient cancer. If no expression / activity of protein A is detected in cancer cells, or if expression / activity of protein A is detected at a lower level than in HR non-deficient control cells, the cancer of the cancer patient is indicated to be a homologous recombination deficient cancer. If protein A is a protein whose expression in cells can be detected as a signal, if no signal of protein A is detected in cancer cells, or if a signal lower than the signal of protein A in the HR non-deficient control is detected, the cancer of the cancer patient is indicated to be a homologous recombination deficient cancer. If protein A is a protein that has the effect of reducing cell viability, if the viability of cancer cells is not reduced, or is higher than the viability of HR non-deficient control cells, the cancer of the cancer patient is indicated to be a homologous recombination deficient cancer. In another embodiment, protein A is a protein whose expression in cells can be detected as a signal, and the introduction of the nucleic acid construct into the cancer cell is performed by administering the nucleic acid construct to a cancer patient, and whether or not a signal of protein A is detected from the cancer lesion is examined. Among the usage modes described below, (2) is a specific example of this embodiment. In this embodiment, the cancer patient is typically a solid cancer patient. The nucleic acid construct is preferably administered locally into the tumor of the cancer patient or near the tumor. From the viewpoint of detecting a signal of protein A from the cancer lesion in the body of the cancer patient, it is preferable to adopt a protein that generates a signal without reacting with a substrate, such as a fluorescent protein, as protein A. If a signal of protein A is detected from the cancer lesion (for example, if a signal of the same level as that of a non-cancerous site near the tumor is detected), it is indicated that the cancer of the patient is not a homologous recombination-deficient cancer. If a signal of protein A is not detected from the cancer lesion (for example, if the signal from the cancer lesion is clearly lower than the signal from the non-cancerous site near the tumor), it is indicated that the cancer of the patient may be a homologous recombination-deficient cancer. In this embodiment, for patients diagnosed with a possible homologous recombination deficient cancer, cancer cells may be obtained and the nucleic acid construct introduced to confirm homologous recombination deficient cancer. In yet another embodiment, protein A is a secretory protein whose expression in cells can be detected (preferably as a signal), and the introduction of the nucleic acid construct into the cancer cells is performed by administering the nucleic acid construct to a cancer patient, and the activity of protein A in blood separated from the patient after administration of the nucleic acid construct is measured. Among the usage embodiments described below, (4) is a specific example of this embodiment. In this embodiment, the cancer patient is typically a solid cancer patient. The nucleic acid construct is preferably administered locally into the tumor of the cancer patient or near the tumor. If the patient's cancer is not deficient in homologous recombination, secretory protein A is expressed from the protein A gene reproduced in the cancer cell and secreted outside the cancer cell, so that the activity of protein A can be detected using a blood sample from the cancer patient. Since the activity of protein A secreted into the blood is detected in vitro, a protein that generates a signal by reacting with a substrate can also be preferably used, and a typical example is secretory luciferase. The gene sequence encoding a secretory protein such as secretory luciferase may be a known secretory protein coding sequence as it is, or may be prepared by adding a sequence encoding a secretory signal to the protein coding sequence. As a negative control sample, for example, a blood sample taken from a patient before administration of a nucleic acid construct can be used. As a positive control, for example, a culture supernatant obtained by introducing a nucleic acid construct into a cell line having normal homologous recombination activity (details are the same as those of the HR non-deficient control cells described above) and culturing the cell line may be used. If protein A activity is detected in the blood, it is indicated that the patient's cancer is not a homologous recombination deficient cancer. If protein A activity is not detected in the blood, it is indicated that the patient's cancer may be a homologous recombination deficient cancer. In this embodiment, for a patient diagnosed with a possibility of a homologous recombination deficient cancer, cancer cells may be collected and a nucleic acid construct may be introduced to confirm that the cancer is a homologous recombination deficient cancer. By using the diagnostic technique for homologous recombination deficient cancer according to the present invention, a cancer patient diagnosed with a homologous recombination deficient cancer can be preferably administered a DNA damaging anticancer drug such as a PARP inhibitor or a platinum agent for cancer treatment. A cancer patient diagnosed with a cancer other than a homologous recombination deficient cancer can be preferably administered an anticancer drug other than a DNA damaging anticancer drug. Examples of usage of the diagnostic agent for homologous recombination deficient cancer include, but are not limited to, the following: (1) Cancer cells taken from a cancer patient are cultured, and the nucleic acid construct of the present invention is introduced therein to examine whether a signal of protein A can be detected (or whether a cytocidal effect of protein A can be observed). If desired, non-cancer cells (such as peripheral blood cells) taken from the same cancer patient may be used as a control. When protein A is luciferase, a luciferase substrate is added to perform a detection reaction. When protein A is a fluorescent protein, a fluorescent signal may be detected using a luminometer or the like. When a signal of protein A is detected (especially when a signal of the same level as that of the control non-cancer cells is detected), it can be determined that the cancer of the patient is not deficient in homologous recombination. When a signal of protein A is not detected or a signal lower than that of the control non-cancer cells is detected, it can be determined that the cancer of the patient is deficient in homologous recombination. Methods for measuring cell viability are well known in the art and can be easily measured using commercially available kits, etc. If the viability of cancer cells into which a nucleic acid construct has been introduced decreases (particularly if it decreases to the same extent as that of control non-cancer cells), it can be determined that the patient's cancer is not deficient in homologous recombination. If no decrease in the viability of cancer cells is observed, or if the decrease in cell viability is more gradual than that of control non-cancer cells, it can be determined that the patient's cancer is deficient in homologous recombination. (2) The nucleic acid construct of the present invention is administered to a patient, and whether or not a signal of protein A is detected from the cancer lesion is examined. If a signal is detected, it can be determined that the patient's cancer is not deficient in homologous recombination, and if no signal is detected, it can be determined that the patient's cancer may be deficient in homologous recombination. In this embodiment, a protein whose intracellular expression can be detected as a signal is used as protein A, but it is necessary to adopt a protein with sufficiently low biotoxicity. The nucleic acid construct is preferably administered to the patient by local administration into the tumor or near the tumor. (3) Cancer cells present in the blood are isolated from the patient and concentrated as necessary, and the nucleic acid construct of the present invention is introduced into the cells to examine whether a signal of protein A is detected (or whether a cytocidal effect of protein A is observed) (in the case of leukemia and CTC use). (4) A nucleic acid construct employing a gene sequence encoding secreted luciferase as the gene sequence encoding protein A is locally administered into or near a patient's tumor, and then a blood sample is collected and examined for the presence or absence of a luciferase reaction in the blood sample. If the patient's cancer is not deficient in homologous recombination, secreted luciferase is produced from the luciferase gene reproduced in the cancer cells and secreted outside the cells, making it possible to detect the luciferase reaction in the blood sample. If the patient's cancer is deficient in homologous recombination, secreted luciferase is not produced, and therefore no luciferase reaction is detected in the blood sample. The nucleic acid construct of the present invention can also be used as a detection agent for homologous recombination restored cancer cells, for example. Homologous recombination restored cancer is a cancer that was initially deficient in homologous recombination but has restored homologous recombination activity as a result of treatment or the like, and a typical example is a cancer that has acquired resistance to treatment with a PARP inhibitor (PARP inhibitor resistant cancer). The term homologous recombination restored cancer also includes cancer in which BRCA1 deficiency has been restored (reversion mutation) and BRCA1 has been normalized. In this use mode, a gene sequence encoding a protein whose expression in cells can be detected can be preferably used as the gene sequence encoding protein A. The specific use mode is the same as the use mode of the diagnostic agent for homologous recombination deficient cancer, but the target cancer patients may mainly be patients who are continuing PARP inhibitor treatment or patients who have been diagnosed with homologous recombination deficient cancer in the past. The nucleic acid construct of the present invention may be introduced into the cancer cells of a patient in the same manner as the above-mentioned diagnostic technique for homologous recombination deficient cancer, and the expression of protein A, preferably the activity of protein A, may be measured. As in the method for diagnosing a homologous recombination-deficient cancer, the method for detecting a homologous recombination-restored cancer cell may use a control cell (HR non-deficient control cell or HR deficient control cell, or both). When the expression / activity of protein A is detected, it can be determined that a homologous recombination-restored cancer cell has been detected. As a method for using the detection agent, for example, the homologous recombination activity in a homologous recombination-deficient cancer cell before the homologous recombination is restored is measured in advance using a homologous recombination activity measurement reagent / kit according to the present invention, and the measured value is compared to determine whether or not the homologous recombination has been restored. When the homologous recombination activity is increased compared to the past measured value, that is, when the expression level / activity level of protein A is increased compared to the expression level / activity level of protein A in a cancer cell previously measured in the cancer patient using the nucleic acid construct of the present invention, it is indicated that the homologous recombination has been restored. For patients diagnosed with a homologous recombination-restored cancer, it is possible to take measures such as discontinuing the administration of a PARP inhibitor and switching to treatment with another anticancer drug (for patients continuing PARP inhibitor treatment).As one of the options for other anticancer drugs, the therapeutic agent for homologous recombination-restoring cancer of the present invention described below can be mentioned. For patients diagnosed as not having homologous recombination-restoring cancer, treatment with the same anticancer drug may be continued or may be switched to treatment with another anticancer drug. The nucleic acid construct of the present invention can also be used as a companion diagnostic agent for predicting the effect of an anticancer drug on a homologous recombination-deficient cancer, for example. An example of the anticancer drug can be a DNA-damaging anticancer drug including a PARP inhibitor. A specific example of the use as a companion diagnostic agent is the same as the above-mentioned diagnostic agent for a homologous recombination-deficient cancer. As the gene sequence encoding protein A, a gene sequence encoding a protein whose expression in cells can be detected can be preferably used, but a gene sequence encoding a protein having an effect of reducing the survival rate of cells can also be used. When a patient is diagnosed with a homologous recombination-deficient cancer, it can be determined that the patient is likely to obtain a desired anticancer effect by a DNA-damaging anticancer drug such as a PARP inhibitor. Therefore, a DNA-damaging anticancer drug can be preferably administered to such a cancer patient. DNA damaging anticancer drugs include inhibitors of proteins that are related to homologous recombination as synthetic lethal (or synthetic growth retardation), and drugs that induce DNA damage that is repaired by homologous recombination. Examples of the former include inhibitors of proteins such as PARP, PolQ, and RAD52, and examples of the latter include camptothecin, cisplatin, and PARP inhibitors, but are not limited to these. Specific examples of PARP inhibitors include, but are not limited to, olaparib, rucaparib, niraparib, veliparib, talazoparib, PJ34 (Visochek et al., Oncotarget, 2019, Vol. 10, (No. 58), pp: 6269-6282), NU1025 (CAS 90417-38-2), and the like. As PolQ inhibitors, novobiocin (Zhou et al., Nature Cancer volume 2, pages 598-610 (2021)) and ART558 (Zatreanu et al., Nature Communications volume 12, Article number: 3636 (2021)) have been reported to have PolQ-specific inhibitory activity. In addition, PolQ inhibitors can be obtained, for example, by the method described in JP 2017-201978 A. PolQ inhibitors also include various PolQ inhibitors that will be discovered in the future by this method or a different method. Examples of RAD52 inhibitors include, but are not limited to, AICAR (CAS 2627-69-2), AICAR 50 phosphate (ZMP), 6-Hydroxy-DL-DOPA (CAS 21373-30-8), D-103, D-G23, (-)-Epigallocatechin, EGC (CAS 490-46-0), and NP-004255 (CAS 23094-69-1) (Hengel SR et al. Cell Chem Biol. 2017 Sep 21;24(9):1101-1119.). Further examples of DNA-damaging anticancer drugs include DDR-related inhibitors that target synthetic lethality (see, e.g., Gourley C et al. J Clin Oncol. 2019 Sep 1;37(25):2257-2269., Gourley C et al. J Clin Oncol. 2019 Sep 1;37(25):2257-2269, and Ashworth A and Lord CJ. Nat Rev Clin Oncol. 2018 Sep;15(9):564-576.). Specific examples include, but are not limited to, CBP-501, Prexasertib, GDC-0575, SRA-737, which target CHK1 / 2; AZD-1775, which targets WEE1; AZD-6738, M-4344, M6620 (VX-970), which target ATR; CC-115, LY-3023414, AsiDNA, M-3814, which target DNA-PK; and AZD-0156, which targets ATM (Gourley C et al., 2019 (supra)). Specific examples of DNA damaging anticancer drugs include platinum preparations (DNA synthesis inhibition) such as cisplatin, carboplatin, and oxaliplatin; pyrimidine drugs (DNA synthesis inhibition) such as fluorouracil and gemcitabine; camptothecin drugs (DNA synthesis inhibition) such as irinotecan and topotecan; epipodophyllotoxin drugs (DNA synthesis inhibition) such as etoposide; anthracycline drugs (DNA synthesis inhibition) such as doxorubicin, epirubicin, and pirarubicin; alkylating agents (DNA synthesis inhibition) such as cyclophosphamide and ifosfamide; and vinca alkaloid drugs (cell division inhibition) such as vinblastine, vincristine, vindesine, and vinorelbine, and taxane drugs (apoptosis inducers) such as paclitaxel and docetaxel, which may also be included in the term DNA damaging anticancer drugs. However, DNA damaging anticancer drugs are not limited to the above examples. The nucleic acid construct of the present invention can also be used as a therapeutic agent for homologous recombination recovery cancer, for example. The definition of homologous recombination recovery cancer is as described above, and includes cancer that has acquired resistance to treatment with a PARP inhibitor (PARP inhibitor-resistant cancer) and cancer in which BRCA1 deficiency has been restored (reversion mutation) and BRCA1 has been normalized. In this mode of use, the gene sequence encoding protein A is a gene sequence encoding a protein that has the effect of reducing cell viability. According to the nucleic acid construct of the present invention, cancer cells having homologous recombination activity can be killed by the action of protein A, so that homologous recombination recovery cancers such as PARP inhibitor-resistant cancers, for which there is currently no treatment method, can be treated. The nucleic acid construct of the present invention can also be used, for example, to determine or predict whether a genetic mutation (particularly a mutation in an HR-related gene) identified in a cancer patient is a pathogenic mutation that impairs homologous recombination activity. When a mutation is identified in a gene X (preferably an HR-related gene) in the cancer cells of a cancer patient, an expression vector expressing the mutant gene X having the same mutation is first constructed. The mutant gene X can be obtained by extracting mRNA from the cancer cells collected from the cancer patient and performing reverse transcription PCR using a primer set targeting the gene X. In addition, an expression vector expressing a wild-type gene X without a mutation is also prepared. The wild-type gene X can be prepared from a known cell line having normal homologous recombination activity (see the above-mentioned HR-non-deficient control cells for details) or from the non-cancer cells of the patient. If an expression vector expressing the wild-type gene X is already known, a known expression vector can be used without newly constructing one. Next, a mutant gene X expression vector and a wild-type gene X expression vector are introduced into the cell lacking gene X, and further, the nucleic acid construct of the present invention is introduced. The introduction of each expression vector and the introduction of the nucleic acid construct may be performed in any order. Gene X-deficient cells can be prepared by knocking out gene X in cells having normal homologous recombination activity. The technology for knocking out a specific gene has been established, and a person skilled in the art can easily prepare gene X knockout cells using well-known techniques. Alternatively, if there is a known cell line that is known to be completely deficient in the function of gene X, such a known cell line may be used. Next, the expression of protein A is measured in cells into which each expression vector and nucleic acid construct has been introduced. In addition, the expression of protein A may also be measured in gene X-deficient cells into which an empty vector without an insert has been introduced, or in gene X-deficient cells into which a vector has not been introduced and the nucleic acid construct of the present invention has been introduced. As described above, the measurement of the expression of protein A is preferably the measurement of the activity of protein A. Based on the expression level of protein A, it can be determined whether the mutation of gene X is a pathological mutation (a mutation that causes a loss of homologous recombination activity). This determination can be performed according to the same criteria as in the above-mentioned technique for measuring homologous recombination activity. If the expression level / activity level of protein A in cells into which mutant gene X has been introduced is lower than the expression level / activity level of protein A in cells into which wild-type gene X has been introduced, it is indicated that the mutation of gene X is a pathological mutation that impairs homologous recombination activity. In this pathological mutation prediction technique, a protein whose intracellular expression can be preferably detected as a signal can be preferably used as protein A, but it is also possible to use a protein that has the effect of reducing cell viability. When the agent of the present invention is administered to a patient, the administration route may be oral or parenteral, but parenteral administration such as intravenous administration, intraarterial administration, subcutaneous administration, and intramuscular administration is generally preferred. It may be administered systemically, intratumorally or near a tumor, or to a lymph node associated with a tumor. However, in the case of a therapeutic agent for cancer that recovers from homologous recombination, local administration into a tumor or near a tumor is preferred, since systemic administration will have a cytocidal effect on normal non-cancer cells that have homologous recombination activity. The administration of an agent containing the nucleic acid construct of the present invention as an active ingredient means administration of a first nucleic acid molecule and a second nucleic acid molecule, and these nucleic acid molecules are preferably administered simultaneously or sequentially. As described above, when administered sequentially, either nucleic acid molecule may be administered first. The dosage is not particularly limited, but may be about 1 pg to 10 g, for example about 0.01 mg to 100 mg, as the total amount of nucleic acid molecules (first nucleic acid molecule + second nucleic acid molecule) per day for a patient. The dosage of the first nucleic acid molecule and the second nucleic acid molecule may be, in terms of molar ratio, first nucleic acid molecule:second nucleic acid molecule=about 1:0.001-1000, about 1:0.01-100, or about 1:0.1-10, or may be about 1:0.2-5, or about 1:0.5-2. The agent of the present invention may be administered once a day, or in several divided doses. It may be administered every day, or every few days or weeks. The dosage form of the agent of the present invention administered to a patient is not particularly limited, and can be formulated by appropriately mixing additives such as pharma- ceutically acceptable carriers, diluents, excipients, etc. with the nucleic acid construct of the present invention according to each administration route. Examples of the formulation form include parenteral preparations such as drip infusions, injections, suppositories, and inhalants, and oral preparations such as tablets, capsules, granules, powders, and syrups. Formulation methods and usable additives are well known in the field of pharmaceutical preparations, and any method and additive can be used. The agent of the present invention may be in the form of a set of a preparation containing a first nucleic acid molecule and a preparation containing a second nucleic acid molecule, or in the form of a preparation containing both the first nucleic acid molecule and the second nucleic acid molecule. In the case of diagnostic agents, companion diagnostic agents and detection agents used in vitro, the amount used when treating cells may be, for example, about 10 pg to 1 mg, for example about 0.1 ng to 100 μg, as the total amount of nucleic acid molecules (first nucleic acid molecule + second nucleic acid molecule) per 1 million cells. The ratio of the amount of the first nucleic acid molecule to the second nucleic acid molecule used may be, in molar ratio, about 1:0.001 to 1000, about 1:0.01 to 100, or about 1:0.1 to 10, or may be about 1:0.2 to 5, or about 1:0.5 to 2. The dosage form may be a liquid agent to which additives useful for the stability of the nucleic acid construct are added as desired, or a powdered agent obtained by lyophilizing the nucleic acid construct. In the case of an agent for in vitro use, it may be in the form of a set of a preparation containing a first nucleic acid molecule and a preparation containing a second nucleic acid molecule, or it may be in the form of a preparation containing both the first nucleic acid molecule and the second nucleic acid molecule. When the nucleic acid construct of the present invention is used as a therapeutic agent, a diagnostic agent, etc., the target cancer patients include cancer patients of various mammals such as humans, dogs, cats, ferrets, hamsters, mice, rats, horses, pigs, etc. The nucleic acid construct of the present invention can be applied to various animal species without changing the configuration, except for changing the origin of the promoter as necessary. The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples. In the following examples, the size of each DNA fragment may be expressed excluding the stop codon added to the 3' end. Example A: Split DR-Nluc construct (co-transfected with SceNluc + iNluc) A-1. Vector construction (Figure 1) A-1-1. Split-type DR-Nluc (middle and bottom rows of Figure 1) The 513 bp SceNluc fragment (SEQ ID NO: 25; DNA in which the region from positions 223 to 243 in the Nluc gene sequence (SEQ ID NO: 23) was replaced with a stop codon (TGA) + I-SceI recognition sequence, with a stop codon added to the 3' end) used in pCMV-SceNluc and the 459 bp iNluc fragment (SEQ ID NO: 28; the region from positions 16 to 474 in the Nluc gene (SEQ ID NO: 23), with a stop codon added to the 3' end) used in pUC-iNluc were amplified by PCR using pNL1.1[Nluc] Vector (Promega, Madison, WI, USA) as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) or Tks Gflex (trade name) DNA Polymerase (Takara Bio) were used as the polymerase for PCR. The primers used are shown in Table 1 below. The SceNluc fragment was amplified by dividing it into two fragments, the 5'SceNluc fragment and the 3'SceNluc fragment. The 5'SceNluc fragment was amplified using Nluc-Fw (SEQ ID NO: 1) and Sce-5'Nluc-Rv (SEQ ID NO: 2), with a sequence for the In-Fusion reaction added to the 5' end of the 5'SceNluc fragment, and a DNA fragment with a structure in which a "stop codon (TGA) + I-SceI recognition sequence" was added to the 3' end was amplified. The 3'SceNluc fragment was amplified using Sce-3'Nluc-Fw (SEQ ID NO: 3) and Nluc-Rv (SEQ ID NO: 4), with a DNA fragment with a structure in which a "stop codon (TGA) + I-SceI recognition sequence" was added to the 5' end of the 3'SceNluc fragment, and a stop codon (TAA) and a sequence for the In-Fusion reaction were added to the 3' end. In the reading frame of the SceNluc fragment (SEQ ID NO: 25), there are a stop codon TGA introduced upstream of the I-SceI recognition sequence and a stop codon TAA present in the I-SceI recognition sequence. The amino acid sequences encoded by positions 1 to 225 (the region up to the first stop codon) and positions 235 to 516 (the region downstream of the second stop codon) of the SceNluc fragment are shown in SEQ ID NOs: 26 and 27, respectively. iNluc-Fw (SEQ ID NO: 5) and iNluc-Rv (SEQ ID NO: 6) were used to amplify the iNluc fragment, and a DNA fragment was amplified in which a stop codon (TGA) was linked to the 3' end of the iNluc fragment and sequences for the In-Fusion reaction were added to both ends. The iNluc fragment amplified by this PCR is a DNA fragment (SEQ ID NO: 30) with a structure having an additional sequence of 23 bp at the 5' end and 22 bp at the 3' end of the 459 bp Nluc coding region. Next, pIRES (Takara Bio, Figure 2) was digested with NheI and XbaI to remove the IRES region and recover a fragment containing the CMV promoter and polyA addition signal. Using the In-Fusion HD Cloning Kit (Takara Bio), the 5'SceNluc fragment and the 3'SceNluc fragment were inserted downstream of the CMV promoter of the recovered fragment to obtain pCMV-SceNluc, which is a ligation of [CMV promoter]-[SceNluc fragment]-[poly A addition signal]. pUC-iNluc was obtained by digesting pUC19 (Takara Bio, Fig. 3) with SmaI in the multicloning site and inserting the iNluc fragment using the DNA Ligation Kit <Mighty Mix> (Takara Bio). pCMV-SceNluc was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK), then cut with I-SceI (New England Biolabs, Ipswich, MA, USA) to linearize it, and then used for transfection. pUC-iNluc was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK), and then either circularized or cut with AhdI (New England Biolabs) to linearize it, and then used for transfection. Note that when pUC-iNluc is cut with AhdI, it becomes a linear DNA with additional sequences of 1.3 kbp at the 5' end and 1.4 kbp at the 3' end of the iNluc fragment. When PCR-amplified iNluc fragments were transfected, they were purified using the Wizard SV Gel and PCR Clean-Up System (Promega) before use. A-1-2. Integrated DR-Nluc (pCMV-DR-Nluc, Figure 1, upper panel) The pCMV-SceNluc prepared in A-1-1 was digested with BamHI, and the PCR-amplified iNluc fragment was inserted downstream of polyA using the In-Fusion HD Cloning Kit (Takara Bio) to obtain the integrated DR-Nluc construct pCMV-DR-Nluc, in which [CMV promoter]-[SceNluc fragment]-[polyA addition signal]-[iNluc fragment] are linked. The constructed plasmid was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK), and linearized by cutting with I-SceI (New England Biolabs, Ipswich, MA, USA) before being used for transfection. A-1-3. shRNA expression vector The target sequence used in the shRNA expression vector (pshRad51) for knocking down RAD51 was designed using Enhanced siDirect (registered trademark) (RNAi, Inc.). The control shRNA expression vector (pshControl) used a sequence previously reported (Bryant et al. (2010) A molecular network for de novo generation of the apical surface and lumen. Nat. Cell Biol. 11: 1035-1045). Next, single-stranded oligo DNA (top strand and bottom strand) arranged in the order of [ligation linker sequence]-[target sequence (sense)]-[loop sequence]-[target sequence (antisense)]-[terminator sequence]-[ligation linker sequence] was annealed to obtain a double-stranded oligo DNA. The sequences of the single-stranded oligo DNA used are shown in Table 1 below. The shRNAs targeted against RAD51 were shRad51-top (SEQ ID NO: 7) and shRad51-btm (SEQ ID NO: 8), and the control shRNAs were shControl-top (SEQ ID NO: 9) and shControl-btm (SEQ ID NO: 10). Finally, pBAsi-hU6 Neo DNA (Takara Bio) was digested with BamHI and HindIII to excise a fragment containing the U6 promoter, and double-stranded oligo DNA was inserted using DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain pshRad51 and pshControl. pshRad51 and pshControl were purified using Qiagen Plasmid Plus Midi Kit (Qiagen KK) and then used in transfection in their circular form. A-2. Cells, gene transfection, and luciferase assay A-2-1. Cells and gene transfer The human pre-B cell line Nalm-6 and its derivatives were cultured in 5% CO 2The cells were cultured at 37°C in an incubator (So et al. (2004) Genetic interactions between BLM and DNA ligase IV in human cells. J. Biol. Chem. 279: 55433-55442). RAD54 / RAD54B double-deficient cells were obtained as previously reported (Saito S, Kurosawa A, Adachi N. Mechanistic basis for increased human gene targeting by promoterless vectors-roles of homology arms and Rad54 paralogs. FEBS J. 2017 Sep;284(17):2748-2763. doi: 10.1111 / febs.14137.). Nalm-6 cells and their derivatives were transfected by electroporation as previously described (Saito et al. (2017) Dual loss of human POLQ and LIG4 abolishes random integration. Nat. Commun. 8: 16112). Human breast cancer cell line MDA-MB-436 (ATCC, Manassas, VA, USA) was cultured in 5% CO 2 The cells were cultured in an incubator at 37°C. MDA-MB-436 was cultured in Eagle's MEM (Nissui Pharmaceutical, Tokyo, Japan) supplemented with calf serum (Global Life Science Technologies Japan, Tokyo, Japan, 50 ml added per 500 ml of Eagle's MEM), L(+)-glutamine (Fujifilm Wako Pure Chemical Industries, Osaka, Japan, added to a final concentration of 2 mM), MEM non-essential amino acid solution (Fujifilm Wako Pure Chemical Industries, added to a final concentration of 1 mM), 100 mmol / L sodium pyruvate solution (Fujifilm Wako Pure Chemical Industries, added to a final concentration of 1 mM), vitamin B12 (Sigma-Aldrich, St. Louis, MO, USA, added to a final concentration of 0.15 μM), and 2-mercaptoethanol (Fujifilm Wako Pure Chemical Industries, added to a final concentration of 50 μM). The human fibrosarcoma-derived cell line HT1080 (Fermentation Research Institute, Osaka, Japan) was incubated at 4°C for 12 h at 5% CO 2 The cells were cultured at 37°C in an incubator (Saito et al. (2015) Construction and applications of exon-trapping gene-targeting vectors with a novel strategy for negative selection. BMC Res. Notes 8: 278). Transfection was performed with jetPEI (Polyplus-transfection, Illkirch, France) according to the manufacturer's protocol. A-2-2. Luciferase assay The luciferase assay was carried out as follows. 2 x 10 6 Each construct (1 μg) was transfected into 5 x 10 Nalm-6 cells (wild type, RAD54 / RAD54B double-deficient homologous recombination-deficient type). 5 The cells were seeded into a 12-well dish at 100 cells / ml, and luciferase activity (RLU value) was measured immediately after gene transfection, and 4 hours, 8 hours, and 24 hours later using the Nano-Glo Luciferase Assay System (Promega). Luciferase assays in MDA-MB-436 cells (human breast cancer cell line, BRCA1 deficient) and HT1080 cells (human fibrosarcoma cell line, with normal homologous recombination activity) were performed as follows. 4 The cells were seeded in a 24-well dish, cultured overnight, and then transfected with each construct (1 μg). Four hours after gene transfection, the cells were counted and luciferase activity (RLU value) was measured using the Nano-Glo Luciferase Assay System (Promega). A-3. Results The results of measuring luciferase activity over time after construct introduction in wild-type and RAD54 / RAD54B double-deficient Nalm-6 cells are shown in Figure 4. pCMV-Nluc is a cell (positive control) into which a construct expressing a normal Nluc gene under the control of a CMV promoter was introduced. In wild-type Nalm-6 cells into which an integrated or split construct was introduced, luciferase activity increased shortly after gene introduction and reached a plateau at about 8 hours. In RAD54 / RAD54B double-deficient strains (cells that have lost the ability for homologous recombination), the luciferase activity detected after construct introduction was lower than that of the wild-type, and the RLU / cell value after 24 hours was 1 / 10 or less that of the wild-type for both the integrated and split constructs. When an integrated or split DR-Nluc construct was introduced into cells with homologous recombination activity, a normal Nluc gene was generated by homologous recombination between the SceNluc fragment and the iNluc fragment, which is thought to result in transient expression of the Nluc protein. The results of luciferase assay in Nalm-6 cells (comparison between wild type and homologous recombination-deficient strain) are shown in Figure 5. The relative values ​​are shown with the RLU value in cells transfected with pCMV-Nluc taken as 100. In cells with homologous recombination activity, homologous recombination occurs between the SceNluc fragment and the iNluc fragment in the integrated construct, or between the split constructs (between pCMV-SceNluc and pUC-iNluc or iNluc), reproducing the normal Nluc gene sequence, and luciferase activity is detected. On the other hand, in cells deficient in homologous recombination, reproducing the normal Nluc gene sequence by homologous recombination does not occur, so luciferase activity is not detected (detection level of luciferase activity is reduced), which is thought to result in a difference in luciferase activity. In fact, as shown in Figure 5, although there were differences depending on the structure of the construct, the luciferase activity was lower when the integrated or split construct was introduced into the homologous recombination-deficient cells than when the integrated or split construct was introduced into the wild-type cells 4 hours after the introduction of the construct, with the difference being several to several tens of times. This confirmed that the homologous recombination activity of cells can be detected with the split construct as well as with the integrated construct (international application pending, PCT / JP2021 / 005381). The difference in luciferase activity between the presence and absence of homologous recombination activity was greater in DNA carrying the iNluc fragment with a longer terminal additional sequence (pCMV-SceNluc + iNluc vs. pCMV-SceNluc + pUC-iNluc (linear)) and in the circular form rather than the linear form (pCMV-SceNluc + pUC-iNluc (linear) vs. pCMV-SceNluc + pUC-iNluc (circular)), and the ability to detect homologous recombination activity was higher. Figure 6 shows the results of comparing luciferase activity in HT1080 cells (cells with normal homologous recombination activity) and MDA-MB-436 cells (cells lacking homologous recombination activity due to BRCA1 deficiency). The values ​​are relative to the RLU value in cells into which pCMV-Nluc was introduced, which was set at 100. For both constructs, the luciferase activity detected in MDA-MB-436 cells, which are homologous recombination-deficient cells, was lower than that detected in HT1080 cells with normal homologous recombination activity, with a difference of more than 100-fold. The experimental results of RAD51 gene knockdown in Nalm-6 cells are shown in Figure 7. The relative values ​​are shown with the RLU value in cells transfected with pCMV-Nluc taken as 100. It was confirmed that the detected luciferase activity was significantly reduced when the gene expression of RAD51, an essential factor for homologous recombination, was suppressed in both the integrated and split types. This demonstrated that the reaction of the construct of the present invention occurs through homologous recombination. Example B: Split DR-DTA construct (co-transfected with SceDTA + iDTA) B-1. Construction of vector (Figure 8) B-1-1. Split-type DR-DTA (middle and bottom of Figure 8) The 588 bp SceDTA fragment (SEQ ID NO: 33; DNA with a structure in which the region from 133 to 153 in the DT-A gene ORF sequence (SEQ ID NO: 31) was replaced with a "stop codon (TGA) + I-SceI recognition sequence", with a stop codon added to the 3' end) used in pCMV-SceDTA was obtained by PCR amplification using pMC1DT-ApA (KURABO, Osaka, Japan) as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR. The primers used are shown in Table 2 below. The SceDTA fragment was amplified into two fragments, the 5'SceDTA fragment and the 3'SceDTA fragment. DTA-Fw (SEQ ID NO: 11) and Sce-5'DTA-Rv (SEQ ID NO: 12) were used to amplify the 5'SceDTA fragment, and a DNA fragment was amplified in which a sequence for In-Fusion reaction was added to the 5' end of the 5'SceDTA fragment and a "stop codon (TGA) + I-SceI recognition sequence" was added to the 3' end. Sce-3'DTA-Fw (SEQ ID NO: 13) and DTA-Rv (SEQ ID NO: 15) were used to amplify the 3'SceDTA fragment, and a DNA fragment was amplified in which a "stop codon (TGA) + I-SceI recognition sequence" was added to the 5' end of the 3'SceDTA fragment and a stop codon (TGA) and a sequence for In-Fusion reaction were added to the 3' end. Next, pIRES (Takara Bio, Figure 2) was digested with NheI and XbaI to remove the IRES region and recover a fragment containing the CMV promoter and polyA addition signal. Using In-Fusion HD Cloning Kit (Takara Bio), the 5'SceDTA fragment and the 3'SceDTA fragment were inserted downstream of the CMV promoter of the recovered fragment to obtain pCMV-SceDTA, in which the CMV promoter, SceDTA fragment, and poly A addition signal were linked. In the reading frame of the SceDTA fragment (SEQ ID NO: 33), there are a stop codon TGA introduced upstream of the I-SceI recognition sequence, and a stop codon TAA present within the I-SceI recognition sequence. The amino acid sequences encoded by positions 1 to 135 (the region up to the first stop codon) and positions 145 to 591 (the region downstream of the second stop codon) of the SceDTA fragment are shown in SEQ ID NOs: 34 and 35, respectively. The DT-A gene fragment (SEQ ID NO: 31) of 588 bp plus stop codon used in pCMV-DTA was obtained by PCR amplification using pMC1DT-ApA (KURABO, Osaka, Japan) as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the PCR polymerase. DTA-Fw (SEQ ID NO: 11) and DTA-Rv (SEQ ID NO: 14) were used as primers to amplify a DNA fragment with sequences for In-Fusion reaction added to the 5' and 3' ends. Next, pIRES (Takara Bio) was digested with NheI and XbaI to remove the IRES region and recover a fragment containing the CMV promoter and polyA addition signal. The DT-A gene fragment was inserted downstream of the CMV promoter of the recovered fragment using the In-Fusion HD Cloning Kit (Takara Bio) to obtain pCMV-DTA in which the [CMV promoter]-[DT-A gene]-[polyA addition signal] were linked. The iDTA fragment of 417 pb + stop codon (SEQ ID NO: 45; 60 bp of vector sequence upstream of the SceDTA fragment in pCMV-SceDTA + the region from positions 1 to 357 in the DT-A gene (SEQ ID NO: 31) + stop codon) having a homologous region to pCMV-SceDTA was obtained by amplifying it by PCR using pCMV-DTA as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) or Tks Gflex (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR. By using iDTA-Fw (SEQ ID NO: 15) and iDTA-Rv (SEQ ID NO: 16) as PCR primers, an iDTA fragment (SEQ ID NO: 45) having a structure in which a 60 bp vector sequence was added to the 5' end of the base sequence of the region from positions 1 to 357 in the DT-A gene + stop codon (SEQ ID NO: 36) was obtained. Next, pUC19 (Takara Bio, Fig. 3) was digested with SmaI in the multicloning site, and the iDTA fragment was inserted using the DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain pUC-iDTA. pCMV-SceDTA was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK) and linearized by digestion with I-SceI (New England Biolabs, Ipswich, MA, USA) before transfection, whereas pUC-iDTA was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK) and used in its circular form for transfection. B-1-2. Integrated DR-DTA (pCMV-DR-DTA-v2, Figure 8, top) pCMV-SceDTA prepared in B-1-1 was digested with BglII, and the iDTA fragment was inserted upstream of the CMV promoter using the In-Fusion HD Cloning Kit (Takara Bio) to obtain pCMV-DR-DTA-v2, which is a ligation of [iDTA fragment]-[CMV promoter]-[SceDTA fragment]-[poly A addition signal]. The plasmid thus prepared was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK), and linearized by cutting with I-SceI (New England Biolabs) before being used for transfection. B-2. Evaluation of cell killing effect The cell-killing effect in Nalm-6 cells was evaluated as follows. 2 x 10 6 Nalm-6 cells (wild type, RAD54 / RAD54B double-deficient type) were transfected with each construct (1 μg), and the cells were then cultured at 1 x 10 5 The cells were seeded at 1000 cells / ml into a 24-well dish, and cell viability was measured after 96 hours using CellTiter-Glo (CellTiter-Glo® Luminescent Cell Viability Assay, Promega). The cytocidal effect on MDA-MB-436 and HT1080 cells was evaluated as follows. 5 x 10 4Cells were seeded in 24-well dishes, cultured overnight, and then transfected with each construct (1 μg). After 96 hours of culture, cell viability was measured using CellTiter-Glo (CellTiter-Glo® Luminescent Cell Viability Assay, Promega). B-3. ​​Results Figure 9A shows the results of comparing the survival rates 96 hours after construct introduction between the wild-type Nalm-6 cell line and the RAD54 / RAD54B double-deficient line. Figure 9B shows the results of comparing the survival rates 96 hours after construct introduction between the human tumor-derived cell line HT1080, which is not deficient in homologous recombination, and the human tumor-derived cell line MDA-MB-436, which is deficient in homologous recombination activity due to BRCA1 deficiency. It was confirmed that, like the integrated construct, the split construct carrying the suicide gene was also able to selectively kill cells with normal homologous recombination activity. Example C: Split DR-TK construct (co-transfected with SceTK + iTK) C-1. Construction of vector C-1-1. Split-type DR-TK (middle and bottom rows of Figure 10) The 1128 bp SceTK fragment used in pCMV-SceTK (SEQ ID NO: 40; DNA structure in which the region from position 499 to position 519 in the ORF sequence of the HSV-TK gene (SEQ ID NO: 38) was replaced with "a stop codon (TGA) + I-SceI recognition sequence", with a stop codon added to the 3' end) was obtained by amplifying the HSV-TK gene fragment (GenBank: V00470.1, Kobayashi et al. (2001) Decreased topoisomerase IIalpha expression confers increased resistance to ICRF-193 as well as VP-16 in mouse embryonic stem cells. Cancer Lett. 166(1): 71-77) as a template by PCR. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR. The primers used are shown in Table 3 below. The SceTK fragment was amplified by dividing it into two fragments, the 5'SceTK fragment and the 3'SceTK fragment containing a polyA addition signal. The 5'SceTK fragment was amplified using Sce-5'TK-Fw (SEQ ID NO: 17) and Sce-5'TK-Rv (SEQ ID NO: 18) to amplify a DNA fragment in which a sequence for In-Fusion reaction was added to the 5' end of the 5'SceTK fragment and a termination codon (TGA) was added to the 3' end. The 3'SceTK fragment was amplified using Sce-3'TK-Fw (SEQ ID NO: 19) and Sce-3'TK-Rv (SEQ ID NO: 20) to amplify a DNA fragment in which an I-SceI recognition sequence was added to the 5' end of the 3'SceTK fragment and a sequence for In-Fusion reaction was added to the 3' end. Next, pIRES (Takara Bio, Figure 2) was digested with NheI and BamHI to excise a fragment containing the CMV promoter. Using the In-Fusion HD Cloning Kit (Takara Bio), the 5'SceTK fragment and the 3'SceTK fragment were inserted downstream of the CMV promoter of the excised fragment to obtain pCMV-SceTK, in which the [CMV promoter]-[SceTK fragment]-[poly A addition signal] are linked.In the reading frame of the SceTK fragment (SEQ ID NO: 40), there are a stop codon TGA introduced upstream of the I-SceI recognition sequence and a stop codon TAA present in the I-SceI recognition sequence. The amino acid sequences encoded by positions 1 to 501 (the region up to the first stop codon) and positions 511 to 1131 (the region downstream of the second stop codon) of the SceTK fragment are shown in SEQ ID NOs: 41 and 42, respectively. The iTK fragment (1063 bp, SEQ ID NO: 43; region from positions 27 to 1089 in the HSV-TK gene ORF sequence) having homology with pCMV-SceTK was obtained by amplifying the HSV-TK gene fragment as a template by PCR. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the PCR polymerase. By using iTK-Fw (SEQ ID NO: 21) and iTK-Rv (SEQ ID NO: 22) as the PCR primers, a DNA fragment with a structure in which sequences for In-Fusion reaction were added to both ends was amplified. Next, pUC19 (Takara Bio) was digested with SmaI in the multicloning site, and the iTK fragment was inserted using the DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain pUC-iTK. pCMV-SceTK was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK) and linearized by digestion with I-SceI (New England Biolabs, Ipswich, MA, USA) before transfection, whereas pUC-iTK was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK) and used in the circular form for transfection. C-1-2. Integrated DR-TK (pCMV-DR-TK, Figure 10, top) The pCMV-SceTK prepared in B-1-1 was digested with BglII, and the iTK fragment was inserted upstream of the CMV promoter using the In-Fusion HD Cloning Kit (Takara Bio) to obtain pCMV-DR-TK, which is a ligation of [iTK fragment]-[CMV promoter]-[SceTK fragment]-[poly A addition signal]. The plasmid thus prepared was purified using the Qiagen Plasmid Plus Midi Kit (Qiagen KK), and linearized by cutting with I-SceI (New England Biolabs) before being used for transfection. C-2. Evaluation of cell killing effect The cytocidal effect on MDA-MB-436 and HT1080 cells was evaluated as follows. 5 x 10 4 The cells were seeded in a 24-well dish, cultured overnight, and then transfected with each construct (1 μg). After transfection, ganciclovir (GANC; Fujifilm Wako Pure Chemical Industries, Ltd.) was added at a final concentration of 500 nM, and the cells were cultured for 96 hours. The cell viability was then measured using CellTiter-Glo (CellTiter-Glo® Luminescent Cell Viability Assay, Promega). C-3. Results The results of comparing the cell-killing effects of each construct in MDA-MB-436 and HT1080 cells are shown in Figure 11. The values ​​are shown as relative values, with the survival rate of cells not transfected with DNA (No DNA) cultured in the presence of GANC (500 nM) taken as 100%. HSV-TK exerts its cell-killing effect by acting on GANC and FIAU, converting them into toxic substances with DNA synthesis inhibitory activity. When HSV-TK was used as a suicide gene, it was confirmed that both the integrated and split types selectively killed cells with normal homologous recombination activity in the presence of GANC. Example D: Split DR-TK30 construct (co-transfected with SceTK + iTK30) The TK30 gene is a mutant of the HSV-TK gene in which the action of ganciclovir has been enhanced by introducing a mutation into an amino acid near the active center of the HSV-TK gene, and it exhibits stronger cytotoxicity than wild-type HSV-TK (Kokoris MS et al. Gene Ther. 1999, Aug;6(8):1415-1426.). In this example, the construct of the present invention was prepared using the TK30 gene as a suicide gene, and the cytocidal effect was evaluated. D-1. Construction of vector D-1-1. Split type DR-TK30 (Figure 12, bottom) To prepare a construct expressing the TK30 gene, mutations were introduced into each codon of the HSV-TK gene to replace alanine at position 152 with valine, leucine at position 159 with isoleucine, isoleucine at position 160 with leucine, phenylalanine at position 161 with alanine, alanine at position 168 with tyrosine, and leucine at position 169 with phenylalanine (FIG. 13). The HSV-TK gene fragment containing these mutations (TK30 gene, SEQ ID NO: 46) was prepared by artificial gene synthesis from the SphI recognition sequence site 5' upstream of the mutation site (position 386 in the ORF sequence of the HSV-TK gene) to the BspEI recognition sequence site 3' downstream of the mutation site (position 629 in the ORF sequence of the HSV-TK gene) (pTK30). The prepared pTK30 was digested with SphI and BspEI, and a fragment containing the TK30 gene was collected. Next, pCMV-TK was digested with SphI and BspEI to remove positions 391 to 624 in the ORF sequence of the HSV-TK gene to recover a fragment containing the CMV promoter, poly A addition signal, and a part of the HSV-TK gene, and then the TK30 fragment was inserted using a DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain a vector (pCMV-TK30, upper panel of FIG. 12) expressing TK30 in which [CMV promoter]-[TK30 fragment]-[poly A addition signal] are linked. The amino acid sequence of the TK30 protein encoded by the TK30 fragment (SEQ ID NO:46) is shown in SEQ ID NO:47. pCMV-SceTK (lower left in FIG. 12), in which the CMV promoter, SceTK fragment and poly A addition signal were ligated, was prepared as described in C-1-1. The iTK30 fragment (1063 bp, region from positions 27 to 1089 in the ORF sequence of the TK30 gene) having homology with pCMV-SceTK was obtained by PCR amplification using pCMV-TK30 as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR. iTK-Fw (SEQ ID NO: 21) and iTK-Rv (SEQ ID NO: 22) were used as the PCR primers. The 428 bp 5'-terminal and 570 bp 3'-terminal base sequences of the iTK30 fragment amplified by this PCR have 100% homology with the 428 bp 3'-terminal (positions 27 to 454 in SEQ ID NO: 40) and 570 bp 5'-terminal (positions 520 to 1089 in SEQ ID NO: 40) of pCMV-SceTK digested with I-SceI, respectively. Next, pUC19 (Takara Bio) was digested with SmaI in the multicloning site, and the iTK30 fragment was inserted using a DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain pUC-iTK30 (Figure 12, lower right). The nucleotide sequence of the iTK30 fragment and the amino acid sequence of the iTK30 protein encoded by positions 2 to 1063 thereof are shown in SEQ ID NOs: 48 and 49, respectively. D-1-2. Integrated DR-TK30 (pCMV-DR-TK30, middle row of Figure 12) To construct pCMV-DR-TK30, which contains the SceTK gene and the iTK30 gene in one vector, a DNA fragment in which sequences for the In-Fusion reaction were added to both ends of the 1063 bp iTK30 fragment was amplified by PCR using pCMV-TK30 as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR, and iTK-Fw (SEQ ID NO: 21) and iTK-Rv (SEQ ID NO: 22) were used as the primers. Next, pCMV-SceTK30 was digested with BglII, and the iTK30 fragment was inserted upstream of the CMV promoter using In-Fusion Snap Assembly Master Mix (Takara Bio) to obtain pCMV-DR-TK30 in which the [iTK30 fragment]-[CMV promoter]-[SceTK fragment]-[poly A addition signal] were linked. pCMV-TK30, pCMV-SceTK, pUC-iTK30, and pCMV-DR-TK30 were purified using a Qiagen Plasmid Plus Midi Kit (Qiagen KK), and then pCMV-TK30 and pUC-iTK30 were used in circular form, while pCMV-SceTK and pCMV-DR-TK30 were cut with I-SceI (New England Biolabs) to linearize them before being used for transfection. D-2. Cells and gene transfer, evaluation of cell killing effect D-2-1. Cells and gene transfer Culture and transfection of the human pre-B cell line Nalm-6 and its derivatives, and culture of the human breast cancer-derived cell line MDA-MB-436 were performed as described in A-2-1. Olaparib-resistant clones (Olaparib R Olaparib (Selleck Chemicals LLC, Houston, TX, USA) was established by culturing MDA-MB-436 cells in medium containing olaparib (10 nM final concentration) for 2 weeks and isolating the resulting colonies (methods previously reported (Zhou J et al. Nat Cancer. 2021 Jun;2(6): 598-610.)). RIn cells, homologous recombination activity was restored, so MDA-MB-436 cells and Olap cells R A construct that exhibits a large difference in luciferase activity or cell-killing effect (i.e., recombination efficiency) between cells can be evaluated as a construct that is sensitive to the presence or absence of homologous recombination activity. MDA-MB-436 and Olap R Cell transfection was performed with jetPEI (Polyplus-transfection) according to the manufacturer's protocol. D-2-2. Evaluation of cell killing effect 2 x 10 6 Each construct (1 μg) was transfected into 1 x 10 Nalm-6 cells. After the transfection, the cells were cultured in a medium containing ganciclovir (Fujifilm Wako Pure Chemical Industries, Ltd.) at a final concentration of 500 nM for 48 hours, and then each construct (1 μg) was transfected again. The transfected cells were cultured at 1 x 10 5 Cells were seeded at 1000 cells / ml in a 24-well dish, and ganciclovir was added to the medium at a final concentration of 500 nM. After 96 hours of culture, cell viability was measured using CellTiter-Glo (CellTiter-Glo® Luminescent Cell Viability Assay, Promega). MDA-MB-436 cells and Olap R The cell killing effect was evaluated in 5 x 10 cells as follows. 4 The cells were seeded in a 24-well dish, cultured overnight, and then transfected with each construct (1 μg). After culturing the transfected cells for 24 hours, each construct (1 μg) was transfected again, and ganciclovir (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the medium at a final concentration of 500 nM. After culturing for 72 hours, cell viability was measured using CellTiter-Glo (CellTiter-Glo® Luminescent Cell Viability Assay, Promega). D-3. Results The construct constructed in this example was constructed so that a normal TK30 gene (modified HSV-TK gene) would be generated when homologous recombination (HR) occurs between the SceTK fragment and the iTK30 fragment. Figure 14 shows the results of comparing the survival rates of Nalm-6 cells transfected with the constructed construct (Figure 12) and its HR-deficient cells (RAD54 / RAD54B double-deficient strain). The survival rates are shown as relative values ​​when the survival rate of cells not transfected with DNA (No DNA) cultured in the presence of GANC (500 nM) was set at 100%. It was confirmed that when the TK30 gene was used as the suicide gene, both the integrated and split types selectively killed cells with normal homologous recombination activity in the presence of GANC. Figure 15 shows the results of MDA-MB-436 cells (BRCA1-deficient cells) and Olap R The results show a comparison of survival rates after introduction of the construct between MDA-MB-436 cells (MDA-MB-436 cells in which homologous recombination activity was restored) and non-transfected cells (No DNA) cultured in the presence of GANC (500 nM). The survival rates were shown relative to 100%. The introduction of the integrated type hardly reduced the survival rate of MDA-MB-436 cells, but the survival rate of Olap cells was significantly reduced. R The cell viability was significantly decreased in the presence of GANC. R The difference in cell viability was even greater (37-fold). These results confirmed that the TK30 gene can selectively kill normal cells that have undergone homologous recombination in the presence of GANC when used as a suicide gene. The selective killing effect was particularly high when the TK30 gene was used as a suicide gene. Example E: Split DR-CDUPRT construct (co-delivery of SceCDUPRT + iCDUPRT) In this example, the CD::UPRT gene was used as a gene encoding a protein that reduces cell viability. The CD::UPRT gene is a fusion gene of the Fcy1 gene (encoding CD (cytosine deaminase)) derived from budding yeast and the Fur1 gene (encoding UPRT (uracil phosphoribosyltransferase)) derived from budding yeast. CD converts 5-fluorocytosine (5-FC) to 5-fluorouracil by deamination. 5-Fluorouracil is metabolized in cells and converted into a substance that inhibits DNA synthesis and RNA synthesis, thereby inducing cell death (Austin EA and Huber BE. Mol Pharmacol. 1993 Mar; 43(3): 380-387.). UPRT is known to catalyze the metabolic pathway by which 5-fluorouracil is converted into a toxic substance within cells, and when used in combination with CD, it enhances the cytotoxicity of 5-fluorocytosine and provides a bystander effect (Tiraby M et al. FEMS Microbiol Lett. 1998 Oct 1;167(1): 41-49., Bourbeau D et al. J Gene Med. 2004 Dec; 6(12): 1320-1332.). E-1. Construction of vector E-1-1. CD::UPRT expression vector, CD expression vector and UPRT expression vector (top of FIG. 16) To create a construct expressing a fusion gene (CD::UPRT gene) of the budding yeast Fcy1 gene (CD) and the budding yeast Fur1 gene (UPRT), the codons of the CD::UPRT gene were optimized for use in human cells as previously reported (Ho YK et al. Sci Rep. 2020 Aug 31;10(1): 14257). In addition, the recognition sequences of XhoI and XbaI were added 5' upstream and 3' downstream of the CD::UPRT gene, respectively, for cloning using restriction enzymes. The sequence of the CD::UPRT gene (SEQ ID NO: 50; encoding a fusion protein (SEQ ID NO: 51) in which the full length of CD and residues 3 to 216 of UPRT are linked by one alanine residue) was created by artificial gene synthesis (pCDUPRT). The created pCDUPRT was digested with XhoI and XbaI, and a fragment containing the CD::UPRT gene was collected. Next, pIRES (Takara Bio) was digested with XhoI and XbaI to remove the IRES region and recover a fragment containing the CMV promoter and poly(A) addition signal. The CD::UPRT fragment was then inserted using the DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain a CD::UPRT expression vector (pCMV-CDUPRT) in which the CMV promoter, CD::UPRT fragment, and poly(A) addition signal were linked. Next, to prepare constructs expressing CD or UPRT alone, a 477 bp Fcy1 gene (a stop codon (TGA) was added to the 3' end of the region from positions 1 to 474 in the CD::UPRT gene sequence; SEQ ID NO: 52) and a 651 bp Fur1 gene (an initiation codon (ATG) and a serine-encoding codon (TCC) were added to the 5' end of the region from positions 478 to 1122 in the CD::UPRT gene sequence; SEQ ID NO: 54) were amplified by PCR using pCMV-CDUPRT as a template (the nucleotide sequences of the resulting Fcy1 gene fragment and Fur1 gene fragment were codon-optimized for expression in human cells). PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR. The primers used are shown in Table 4 below. For amplification of the Fcy1 gene, Sce-5'CDUPRT-Fw (SEQ ID NO: 60) and CD-Rv (SEQ ID NO: 61) were used to amplify a DNA fragment with a structure in which a sequence for In-Fusion reaction was added to the 5' end of the Fcy1 gene fragment and a stop codon (TGA) and a sequence for In-Fusion reaction were added to the 3' end. Next, pIRES (Takara Bio, Figure 2) was digested with XhoI and XbaI to remove the IRES region and recover a fragment containing the CMV promoter and poly A addition signal, and then the Fcy1 gene fragment was inserted using the DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain a codon-optimized CD expression vector (pCMV-CD) in which the [CMV promoter]-[Fcy1 gene fragment]-[poly A addition signal] were ligated. The Fur1 gene was amplified using UPRT-Fw (sequence number 62) and Sce-3'CDUPRT-Rv (sequence number 63), thereby amplifying a DNA fragment having a structure in which an initiation codon, a codon encoding serine, and a sequence for the In-Fusion reaction were added to the 5' end of the Fur1 gene fragment, and a sequence for the In-Fusion reaction was added to the 3' end.Next, pCMV-CDUPRT was digested with AgeI and XbaI to remove the CD::UPRT gene and recover a fragment containing the CMV promoter and poly A addition signal. The Fur1 gene fragment was then inserted using the DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain a UPRT expression vector (pCMV-UPRT) in which the CMV promoter, Fur1 gene fragment, and poly A addition signal were linked. E-1-2. Split DR-CDUPRT construct (Figure 16, bottom) To prepare a construct that reduces cell viability depending on the homologous recombination activity of cells, a 1120 bp SceCDUPRT fragment (SEQ ID NO: 56; DNA structure in which the region from positions 271 to 300 in the CD::UPRT gene sequence is replaced with "stop codon (TGA) + cgcg + I-SceI recognition sequence", with a stop codon added to the 3' end) and a 1022 bp iCDUPRT fragment (SEQ ID NO: 58; region from positions 46 to 1067 in the CD::UPRT gene sequence) were amplified by PCR using pCMV-CDUPRT as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR. The primers used are shown in Table 4 below. The amplification of the SceCDUPRT fragment was carried out by dividing it into two fragments, the 5'SceCDUPRT fragment and the 3'SceCDUPRT fragment. The 5'SceCDUPRT fragment was amplified using Sce-5'CDUPRT-Fw (SEQ ID NO: 60) and Sce-5'CDUPRT-Rv (SEQ ID NO: 64) to amplify a DNA fragment having a structure in which a sequence for In-Fusion reaction was added to the 5' end of the 5'SceCDUPRT fragment and a "stop codon (TGA) + cgcg + I-SceI recognition sequence" was added to the 3' end. The 3'SceCDUPRT fragment was amplified using Sce-3'CDUPRT-Fw (SEQ ID NO: 65) and Sce-3'CDUPRT-Rv (SEQ ID NO: 63) to amplify a DNA fragment having a structure in which an "I-SceI recognition sequence" was added to the 5' end of the 3'SceCDUPRT fragment and a sequence for In-Fusion reaction was added to the 3' end. Next, pIRES (Takara Bio, Figure 2) was digested with NheI and BamHI to remove the IRES region and polyA addition signal, and a fragment containing the CMV promoter was recovered. Using In-Fusion Snap Assembly Master Mix (Takara Bio, formerly In-Fusion HD Cloning Kit), the 5'SceCDUPRT fragment and the 3'SceCDUPRT fragment were inserted downstream of the CMV promoter of the recovered fragment, to obtain pCMV-SceCDUPRT, in which [CMV promoter]-[SceCDUPRT fragment]-[polyA addition signal] are linked. In the reading frame of the SceCDUPRT fragment (SEQ ID NO: 56), there is a stop codon TGA introduced upstream of the I-SceI recognition sequence, and downstream of that, a frameshift occurs, resulting in multiple stop codons. The amino acid sequence encoded by positions 1 to 270 of the SceCDUPRT fragment (the region up to the first stop codon) is shown in SEQ ID NO: 57. iCDUPRT-Fw (SEQ ID NO: 66) and iCDUPRT-Rv (SEQ ID NO: 67) were used to amplify an iCDUPRT fragment (SEQ ID NO: 58) having homology with pCMV-SceCDUPRT. The 227 bp base sequence at the 5' end and the 767 bp base sequence at the 3' end of the iCDUPRT fragment amplified by this PCR are 100% homologous to the 227 bp base sequence at the 3' end (positions 46 to 272 in SEQ ID NO: 56) and the 767 bp base sequence at the 5' end (positions 299 to 1065 in SEQ ID NO: 56) of pCMV-SceCDUPRT cleaved with I-SceI, respectively. Next, pUC19 (Takara Bio) was digested with SmaI in the multicloning site, and the iCDUPRT fragment was inserted using the DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain pUC-iCDUPRT. The amino acid sequence of the iCDUPRT protein encoded by the iCDUPRT fragment (SEQ ID NO:58) is shown in SEQ ID NO:59. E-1-3. Integrated DR-CDUPRT construct (middle row of Figure 16) To construct pCMV-DR-CDUPRT, in which the SceCDUPRT gene and the iCDUPRT gene are carried in one vector, a DNA fragment in which sequences for In-Fusion reaction were added to both ends of the 1022 bp iCDUPRT fragment was amplified by PCR using pCMV-CDUPRT as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR, and iCDUPRT-Fw (SEQ ID NO: 66) and iCDUPRT-Rv (SEQ ID NO: 67) were used as the primers. Next, pCMV-SceCDUPRT was digested with BglII, and the iCDUPRT fragment was inserted upstream of the CMV promoter using In-Fusion Snap Assembly Master Mix (Takara Bio), to obtain pCMV-DR-CDUPRT in which [iCDUPRT fragment]-[CMV promoter]-[SceCDUPRT fragment]-[poly A addition signal] were linked. All plasmids prepared were purified using a Qiagen Plasmid Plus Midi Kit (Qiagen KK), and then pCMV-CDUPRT, pCMV-CD, pCMV-UPRT, and pUC-iCDUPRT were used in circular form, while pCMV-SceCDUPRT and pCMV-DR-CDUPRT were linearized by cleavage with I-SceI (New England Biolabs) before being used for transfection. E-2. Cells and gene transfer, evaluation of cell killing effect E-2-1. Cells and gene transfer Same as D-2-1. E-2-2. Evaluation of cell killing effect 2 x 10 6 Each construct (1 μg) was transfected into 1 x 10 Nalm-6 cells. After the transfection, the cells were cultured in a medium containing 5-fluorocytosine (Fujifilm Wako Pure Chemical Industries, Ltd.) at a final concentration of 100 μM for 48 hours, and then each construct (1 μg) was transfected again. The transfected cells were cultured at 1 x 10 5 Cells were seeded at 100 cells / ml in a 24-well dish, and 5-fluorocytosine (5-FC) was added to the medium at a final concentration of 100 μM. After 96 hours of culture, cell viability was measured using CellTiter-Glo (CellTiter-Glo® Luminescent Cell Viability Assay, Promega). MDA-MB-436 cells and Olap R The cell killing effect was evaluated in 5 x 10 cells as follows. 4The cells were seeded in a 24-well dish, cultured overnight, and then transfected with each construct (1 μg). After culturing the transfected cells for 24 hours, each construct (1 μg) was transfected again, and 5-fluorocytosine (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the medium at a final concentration of 100 μM. After culturing for 72 hours, cell viability was measured using CellTiter-Glo (CellTiter-Glo® Luminescent Cell Viability Assay, Promega). E-3. Results E-3-1. Confirmation of the effect of CDUPRT, CD or UPRT expression alone To examine whether co-expression of CD and UPRT enhances the cytotoxicity of 5-FC, each construct was introduced into wild-type Nalm-6 cells and the viability was compared. The results are shown in Figure 17. The values ​​are shown as relative values, with the viability of non-transfected cells (No DNA) cultured with 5-FC (100 μM) taken as 100%. Cells transfected with pCMV-CD alone showed a decreased viability in the presence of 5-FC, but transfection with pCMV-UPRT alone did not decrease the cell viability. This result indicates that expression of UPRT alone does not cause cytotoxicity. Next, we investigated the effect of co-expression of CD and UPRT. Compared with pCMV-CD alone, co-transfection of pCMV-CD and pCMV-UPRT showed a cytocidal effect that was approximately 5-fold higher, and transfection of pCMV-CDUPRT (a construct expressing a fusion gene of CD and UPRT) showed a cytocidal effect that was approximately 13-fold higher. These results confirmed that, although the cytocidal effect of 5-FC can be achieved by expressing only CD, an even higher cytocidal effect can be achieved by expressing both CD and UPRT. E-3-2. Cell killing effect The construct constructed in this example was constructed so that a normal CD::UPRT gene would be generated when homologous recombination (HR) occurs between the SceCDUPRT fragment and the iCDUPRT fragment (middle and bottom panels of FIG. 16). The results of comparing the survival rates between Nalm-6 cells transfected with the constructed construct and their HR-deficient cells (RAD54 / RAD54B double-deficient strain) are shown in FIG. 18. The survival rates are shown as relative values ​​when cells not transfected with DNA (No DNA) were cultured in the presence of 5-FC (100 μM), which was taken as 100%. Even when CD::UPRT was used as a suicide gene, it was confirmed that both the integrated and split types selectively killed cells with normal homologous recombination activity in the presence of 5-FC. Figure 19 shows the results of the analysis of MDA-MB-436 cells and Olap cells. R The results are a comparison of the survival rates after transfection of the construct between MDA-MB-436 and Olap cells. The survival rates are shown as relative values, with the survival rate of non-transfected cells (no DNA) cultured with 5-FC (100 μM) taken as 100%. Transfection of the integrated type hardly reduced the survival rate of MDA-MB-436 cells, but the survival rate of Olap cells was significantly reduced. R The cell viability was significantly decreased in the presence of 5-FC (26-fold difference). R The difference in cell viability was even greater (43-fold difference). These results confirmed that the CD::UPRT gene could selectively kill normal cells that had undergone homologous recombination in the presence of 5-FC, even when used as a suicide gene. In particular, the split construct showed a higher selective killing effect than the integrated construct. Example F: Split DR-CeNL construct (co-transfected with SceCeNL + iCeNL) The CeNL (cyan enhanced nano-lantern) gene encodes a fusion protein of the chemiluminescent protein Nluc and the cyan fluorescent protein mTurquoise2 (Goedhart J et al. Nat Commun. 2012, Mar 20;3: 751.). The luminescence intensity of the CeNL protein is approximately twice as high as that of Nluc due to the resonance energy transfer (FRET) that occurs between Nluc and mTurquoise2 (Suzuki K et al. Nat Commun. 2016, Dec 14;7: 13718.). Therefore, by using the sequence of the CeNL gene in the nucleic acid molecule of the present invention, it is possible to measure homologous recombination activity in a shorter time (with higher sensitivity) than when using a chemiluminescent gene alone. Specific examples are shown below. F-1. Construction of vector F-1-1. CeNL expression vector (Figure 20, top) To prepare a construct expressing the CeNL gene, a CeNL gene partial fragment was prepared by artificial gene synthesis in which the XhoI recognition sequence was linked to the 5' upstream of the mTurquoise2 gene (the region from positions 1 to 687 of the mTurquoise2 gene sequence shown in SEQ ID NO: 68) and the region from positions 10 to 79 (including the EcoNI recognition sequence) in the ORF sequence of the Nluc gene was linked to the 3' downstream of the mTurquoise2 gene (pCeNL). The prepared pCeNL was digested with XhoI and EcoNI, and a fragment containing the CeNL gene partial fragment was recovered. Next, pCMV-Nluc was digested with XhoI and EcoNI to remove the untranslated region from the 5' upstream of the Nluc gene and positions 1 to 73 in the ORF sequence of the Nluc gene, and a fragment containing the CMV promoter, poly A addition signal, and a part of the Nluc gene was recovered. A partial fragment of the CeNL gene was inserted into the recovered fragment using the DNA Ligation Kit <Mighty Mix> (Takara Bio), to obtain a vector (pCMV-CeNL) expressing CeNL, in which the [CMV promoter]-[CeNL gene fragment]-[poly A addition signal] were linked. The nucleotide sequence of the CeNL gene and the amino acid sequence of the CeNL protein encoded thereby are shown in SEQ ID NOs: 70 and 71, respectively. The CeNL protein prepared here was a protein with a structure in which residues 1 to 229 of the mTurquoise2 protein (SEQ ID NO: 69) and residues 4 to 171 of the Nluc protein (SEQ ID NO: 24) were linked via LH. F-1-2. Split DR-CeNL construct (Figure 20, bottom) The 1080 bp SceCeNL fragment used in pCMV-SceCeNL (SEQ ID NO: 72; DNA structure in which the region from position 601 to position 749 in the ORF sequence of the CeNL gene is replaced with "stop codon (TGA) + attc + I-SceI recognition sequence", with a stop codon added to the 3' end) was obtained by amplifying pCMV-CeNL by PCR using the template pCMV-CeNL. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR. The primers used are shown in Table 5 below. The SceCeNL fragment was amplified into two fragments, the 5'SceCeNL fragment and the 3'SceCeNL fragment. For the amplification of the 5'SceCeNL fragment, Nluc-Fw (SEQ ID NO: 1) and Sce-5'CeNL-Rv (SEQ ID NO: 78) were used to amplify a DNA fragment in which a sequence for the In-Fusion reaction was added to the 5' end of the 5'SceCeNL fragment and a stop codon (TGA) was added to the 3' end. For the amplification of the 3'SceCeNL fragment, Sce-3'CeNL-Fw (SEQ ID NO: 79) and Nluc-Rv (SEQ ID NO: 4) were used to amplify a DNA fragment in which an "I-SceI recognition sequence" was added to the 5' end of the 3'SceTK fragment and a sequence for the In-Fusion reaction was added to the 3' end. Next, pIRES (Takara Bio, Figure 2) was digested with NheI and XbaI to remove the IRES region and recover a fragment containing the CMV promoter and polyA addition signal. Using the In-Fusion HD Cloning Kit (Takara Bio), the 5'SceCeNL fragment and the 3'SceCeNL fragment were inserted downstream of the CMV promoter of the recovered fragment to obtain pCMV-SceCeNL, in which the [CMV promoter]-[SceCeNL fragment]-[poly A addition signal] were linked. In the reading frame of the SceCeNL fragment (SEQ ID NO: 72), there are the stop codon TGA introduced upstream of the I-SceI recognition sequence and the stop codon TAA present in the I-SceI recognition sequence.The amino acid sequences encoded by positions 1 to 603 (the region from position 1 to the first stop codon TGA), positions 604 to 624 (the region downstream of the first stop codon TGA to the second stop codon TAA), and positions 625 to 1080 (the region downstream of the second stop codon) of the SceCeNL fragment are shown in SEQ ID NOs: 73 to 75, respectively. The iCeNL fragment (SEQ ID NO: 76; 944 bp, region from positions 212 to 1155 in the ORF sequence of the CeNL gene) having homology with pCMV-SceCeNL was obtained by PCR amplification using pCMV-CeNL as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR. iCeNL-Fw (SEQ ID NO: 80) and iCeNL-Rv (SEQ ID NO: 81) were used as the PCR primers. The 390 bp 5'-terminal and 406 bp 3'-terminal base sequences of the iCeNL fragment amplified by this PCR are 100% homologous to the 390 bp 3'-terminal (212 to 601 bp in SEQ ID NO: 72) and 406 bp 5'-terminal (630 to 1035 bp in SEQ ID NO: 72) of pCMV-SceCeNL digested with I-SceI, respectively. Next, pUC19 (Takara Bio) was digested with SmaI in the multiple cloning site, and the iCeNL fragment was inserted using the DNA Ligation Kit <Mighty Mix> (Takara Bio) to obtain pUC-iCeNL. The amino acid sequence of the iCeNL protein encoded by positions 3 to 944 of the iCeNL fragment (SEQ ID NO: 76) is shown in SEQ ID NO: 77. F-1-3. Integrated DR-CeNL construct (middle of Figure 20) To construct pCMV-DR-CeNL, in which the SceCeNL gene and the iCeNL gene are carried in one vector, a DNA fragment in which sequences for the In-Fusion reaction were added to both ends of the 944 bp iCeNL fragment was amplified by PCR using pCMV-CeNL as a template. PrimeSTAR HS (trade name) DNA Polymerase (Takara Bio) was used as the polymerase for PCR, and iCeNL-Fw (SEQ ID NO: 80) and iCeNL-Rv (SEQ ID NO: 81) were used as the primers. Next, pCMV-SceCeNL was digested with BglII, and the iCeNL fragment was inserted upstream of the CMV promoter using In-Fusion Snap Assembly Master Mix (Takara Bio) to obtain pCMV-DR-CeNL in which the [iCeNL fragment]-[CMV promoter]-[SceCeNL fragment]-[poly A addition signal] were linked. pCMV-CeNL, pCMV-SceCeNL, pUC-iCeNL, and pCMV-DR-CeNL were purified using a Qiagen Plasmid Plus Midi Kit (Qiagen KK), and then pCMV-CeNL and pUC-iCeNL were used in circular form, while pCMV-SceCeNL and pCMV-DR-CeNL were cut with I-SceI (New England Biolabs) to linearize them before being used for transfection. F-2. Cells, gene transfection, and luciferase assay F-2-1. Cells and gene transfer Same as D-2-1. F-2-2. Luciferase assay 2 x 10 6 Each construct (1 μg) was transfected into 5 x 10 Nalm-6 cells (wild type). 5The cells were seeded into a 12-well dish at 100 cells / ml, and luciferase activity (RLU value) was measured immediately after gene transfection, and 1, 2, 4, 8, and 24 hours later using the Nano-Glo Luciferase Assay System (Promega). MDA-MB-436 cells and Olap R Luciferase assays were performed in 5 x 10 cells. 4 The cells were seeded in a 24-well dish, cultured overnight, and then transfected with each construct (1 μg). Luciferase activity (RLU value) was measured immediately after transfection, and 1, 2, 4, 8, and 24 hours later using the Nano-Glo Luciferase Assay System (Promega). F-3. Results The constructed construct was designed so that a normal CeNL gene would be generated when homologous recombination (HR) occurred between the SceCeNL and iCeNL fragments (Figure 20). Figure 21 shows the results of measuring luciferase activity over time after construct introduction in wild-type and RAD54 / RAD54B double-deficient Nalm-6 cells (cells deficient in homologous recombination). pCMV-CeNL is a cell (positive control) into which a construct expressing a normal CeNL gene under the control of a CMV promoter was introduced. In wild-type Nalm-6 cells into which an integrated (pCMV-DR-CeNL) or split-type (pCMV-SceCeNL + pUC-iCeNL) construct was introduced, an increase in luciferase activity was observed within 2 hours after gene introduction. On the other hand, in the RAD54 / RAD54B double-deficient strain, both the integrated and split-type luciferase activities were less than 1 / 15 of those in the wild-type. The construct of Example A using Nluc or the construct of this Example F using CeNL were introduced into Nalm-6 cells, and the results of luciferase assay were performed 2 hours and 4 hours later are shown in Figures 22 and 23. The relative values ​​are shown when the RLU value in the cells introduced with pCMV-CeNL or pCMV-Nluc was set to 100. Since the luminescence intensity of the CeNL protein is higher than that of the Nluc protein, it is considered that luciferase activity can be detected in a shorter time period with the construct using CeNL than with Nluc. In fact, as shown in Figure 22, a clear difference in luciferase activity was observed between the wild type and the RAD54 / RAD54B double-deficient strain 2 hours after introduction with the construct using CeNL. In the case of the construct of Example A using Nluc, no increase in luciferase activity was confirmed 2 hours after introduction (right side of Figure 22). Four hours after the introduction of the constructs, differences in luciferase activity between the wild-type strain and the RAD54 / RAD54B double-deficient strain were observed regardless of whether the CeNL or Nluc construct was used (Figure 23). This result confirmed that the homologous recombination activity of cells can be detected using the CeNL construct as well as the Nluc construct, and that the CeNL construct can detect the homologous recombination activity of cells in an even shorter time than the Nluc construct. MDA-MB-436 cells (cells lacking homologous recombination activity due to BRCA1 deficiency) and Olap R The results of comparing luciferase activity between Olap cells (MDA-MB-436 cells in which homologous recombination activity was restored) and the MDA-MB-436 cells in which homologous recombination activity was restored are shown in Figure 24. R In MDA-MB-436 cells, the luciferase activity increased within 2 hours after gene transfection. In contrast, the luciferase activity detected after transfection with either construct was only slightly higher than that detected with Olap. R This was lower than that of cells, with significant differences of more than 10 times for the integrated type and more than 20 times for the divided type. Nluc or CeNL-based constructs were transfected with MDA-MB-436 cells and Olap cells. R The results of luciferase assays performed 2 and 4 hours after introduction into cells are shown in Figures 25 and 26. The values ​​shown are relative values ​​when the RLU value in cells introduced with pCMV-CeNL or pCMV-Nluc was set at 100. As in the example in Nalm-6 cells, it was confirmed that the construct of this Example F using CeNL allowed detection of homologous recombination activity in cells in an even shorter time than the construct of Example A using Nluc.

Claims

1. A nucleic acid construct comprising a set of a first nucleic acid molecule and a second nucleic acid molecule, The first nucleic acid molecule comprises a promoter region and a mutant gene sequence having a cleavage site within a gene sequence encoding a protein, the mutant gene sequence being located downstream of the promoter region; The second nucleic acid molecule is a nucleic acid construct comprising a complementary region composed of a first homologous region and a second homologous region capable of replacing a partial region including the cleavage site in the mutant gene sequence by homologous recombination, the complementary region comprising a base sequence selected from the following (i) to (iii): (i) A continuous subsequence in a gene sequence encoding the protein, the subsequence including the upstream and downstream regions adjacent to the cleavage site in the mutant gene sequence. (ii) A nucleotide sequence that is homologous to a partial sequence of (i) and encodes the same amino acid sequence as said partial sequence. (iii) A contiguous partial sequence in a gene sequence encoding a protein having 80% or more sequence identity with the protein and the same activity as the protein, said partial sequence having homology to the partial sequence of (i).

2. The nucleic acid construct of claim 1 , wherein the mutant gene sequence comprises a stop codon upstream of the cleavage site.

3. The nucleic acid construct according to claim 1 , wherein the base sequences of (ii) and (iii) have a homology of 90% or more with the partial sequence of (i).

4. The nucleic acid construct of claim 1, wherein the second nucleic acid molecule comprises a contiguous subsequence in the gene sequence encoding the protein, the subsequence including the upstream and downstream regions adjacent to the cleavage site in the mutant gene sequence.

5. The nucleic acid construct of claim 2, wherein the second nucleic acid molecule comprises a contiguous subsequence of the gene sequence encoding the protein, the subsequence including the upstream and downstream regions adjacent to the stop codon and cleavage site in the mutant gene sequence.

6. 2. The nucleic acid construct according to claim 1, wherein the length of each of the first and second homologous regions is at least 20 bases.

7. The nucleic acid construct of claim 1 , wherein the first nucleic acid molecule comprises a poly A addition signal downstream of the mutant gene sequence.

8. The nucleic acid construct of claim 1 , wherein the cleavage site is a restriction enzyme recognition site.

9. The nucleic acid construct according to claim 1, wherein the second nucleic acid molecule is a circular nucleic acid molecule further comprising an additional sequence of 1 to 20,000 bases linked to the complementary region, or a linear nucleic acid molecule having an additional sequence of 1 to 10,000 bases linked to at least one of the 5'-terminus and 3'-terminus of the complementary region.

10. The nucleic acid construct according to claim 1 , wherein the first nucleic acid molecule is a circular nucleic acid molecule or a linear nucleic acid molecule obtained by cleaving the circular nucleic acid molecule at the cleavage site.

11. The nucleic acid construct according to claim 1 , wherein the gene sequence is a gene sequence encoding a protein having an effect of reducing cell viability or a protein whose expression in a cell can be detected.

12. The nucleic acid construct according to claim 11 , wherein the gene sequence is a sequence of a suicide gene, a DNA damage-inducing gene, a DNA repair inhibitor gene, a luciferase gene, a fluorescent protein gene, a cell surface antigen gene, a secreted protein gene, or a membrane protein gene.

13. A reagent for measuring homologous recombination activity, comprising the nucleic acid construct according to any one of claims 1 to 12, wherein the gene sequence is a gene sequence encoding a protein whose expression in a cell can be detected.

14. A kit for measuring homologous recombination activity, comprising the measurement reagent according to claim 13.

15. A screening system for a drug that affects homologous recombination activity, comprising the nucleic acid construct according to any one of claims 1 to 12.

16. A diagnostic agent for homologous recombination deficient cancer, comprising the nucleic acid construct according to any one of claims 1 to 12.

17. A detection agent for detecting homologous recombination restored cancer cells, comprising the nucleic acid construct according to any one of claims 1 to 12, wherein the gene sequence is a gene sequence encoding a protein whose expression in a cell can be detected.

18. The detection agent according to claim 17 , wherein the homologous recombination restored cancer cells are PARP inhibitor-resistant cancer cells.

19. A companion diagnostic agent for predicting the effect of an anticancer drug on a homologous recombination deficient cancer, comprising the nucleic acid construct according to any one of claims 1 to 12.

20. The companion diagnostic of claim 19 , wherein the anticancer agent is a DNA damaging anticancer agent.

21. The companion diagnostic of claim 20, wherein the DNA damaging anticancer drug is a PARP inhibitor.

22. A therapeutic agent for homologous recombination restored cancer, comprising the nucleic acid construct according to any one of claims 1 to 12, wherein the gene sequence is a gene sequence encoding a protein having an effect of reducing cell viability.

23. The therapeutic agent according to claim 22, wherein the homologous recombination restored cancer is a PARP inhibitor resistant cancer.

24. 13. Introducing the nucleic acid construct according to any one of claims 1 to 12, wherein the gene sequence is a gene sequence encoding a protein whose expression in a cell can be detected, into a test cell in which homologous recombination activity is to be measured and into a control cell not deficient in homologous recombination and having normal homologous recombination activity; Measuring the expression level of the protein in the test cells and in a control cell that is not deficient in homologous recombination; and Comparing the expression level in the test cells with the expression level in control cells that are not deficient in homologous recombination A method for measuring homologous recombination activity in a test cell, comprising:

25. Introduction of the nucleic acid construct according to any one of claims 1 to 12 into a cell having normal homologous recombination activity, followed by treatment of the cell with an individual compound of the compound group, or introduction of the nucleic acid construct according to any one of claims 1 to 12 into a cell having normal homologous recombination activity, followed by treatment of the cell with an individual compound of the compound group; and Measuring the expression of said protein. A method for identifying candidate drugs that affect homologous recombination activity, comprising:

26. The method according to claim 25, wherein the protein is a protein whose expression in a cell can be detected as a signal, and the method comprises selecting the compound as a candidate inhibitor that inhibits homologous recombination activity when a lower protein signal is detected in a cell treated with the compound than in a cell not treated with the compound, or selecting the compound as a candidate promoter that promotes homologous recombination activity when a protein signal rises earlier or a higher signal is detected in a cell treated with the compound than in a cell not treated with the compound.

27. The method according to claim 25, wherein the protein is a protein having the effect of reducing cell viability, and when a reduction in cell viability is suppressed in cells treated with the compound compared to cells not treated with the compound, the method comprises selecting the compound as a candidate inhibitor that inhibits homologous recombination activity, or when a reduction in viability of cells treated with the compound occurs more quickly than a reduction in viability of cells not treated with the compound, selecting the compound as a candidate promoter that promotes homologous recombination activity.

28. Introducing the nucleic acid construct according to any one of claims 1 to 12 into cancer cells of a cancer patient; and Measuring the expression of said protein.

16. A method for detecting a homologous recombination deficient cancer, comprising:

29. Introducing the nucleic acid construct according to any one of claims 1 to 12 into cancer cells of a cancer patient who is undergoing PARP inhibitor treatment or who has previously been diagnosed with a homologous recombination deficient cancer; and Measuring the expression of said protein.

16. A method for detecting homologous recombination restored cancer cells, comprising:

30. Introducing the nucleic acid construct according to any one of claims 1 to 12 into cancer cells of a cancer patient; and Measuring the expression of said protein. A method for predicting the effect of an anticancer drug on a homologous recombination deficient cancer, comprising:

31. 31. The method of claim 30, wherein the anticancer agent is a DNA damaging anticancer agent.

32. 32. The method of claim 31, wherein the DNA damaging anticancer agent is a PARP inhibitor.

33. A method for predicting whether a genetic mutation identified in a cancer patient is a pathogenic mutation that impairs homologous recombination activity, comprising: constructing an expression vector that expresses a mutant gene having the same mutation as the mutation; preparing an expression vector that expresses a wild-type gene that does not have the mutation; introducing a mutant gene expression vector and a wild-type gene expression vector into the cell lacking the gene, and also introducing the nucleic acid construct according to any one of claims 1 to 12; Measuring the expression of the protein in a cell into which any of the expression vectors and nucleic acid constructs have been introduced; wherein, when the expression level of the protein in a cell into which a mutant gene expression vector has been introduced is lower than the expression level of the protein in a cell into which a wild-type gene expression vector has been introduced, it is indicated that the mutation is a pathogenic mutation that impairs homologous recombination activity.

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