The major hereditary breast cancer susceptibility gene BRCA2 is associated with familial breast and ovarian cancer. BRCA2 plays a role in DNA repair, transcription, cell cycle regulation, maintenance of genomic stability in response to DNA damage, centrosome regulation, and cytokinesis. To further understand the function of BRCA2, we used a yeast two-hybrid method and identified a novel BRCA2-interacting protein, BJ-HCC-20A, which is reported to be a potential cancer-testis antigen. We confirmed the interaction between endogenous BJ-HCC- G enetic factors contribute to an ill-defined proportion of breast cancer incidence, estimated to be approximately 5% of all breast cancer cases.(1) The two major hereditary breast cancer susceptibility genes, BRCA1 and BRCA2, are associated with familial breast and ovarian cancer. Germline mutations in the BRCA1 gene increase the risk of development of early onset breast cancer and ovarian cancer.(2) Alterations in the BRCA2 gene result in an increased risk of development of breast cancer in both women and men, (3,4) and a moderately increased risk of a variety of other cancers, including carcinomas of the ovary, pancreas, prostate, and colon.(5) Because inactivation of both alleles of either BRCA1 or BRCA2 is a key feature in neoplastic development in hereditary cancers, these genes are believed to act as tumor-suppressor genes required for cell growth. BRCA2 plays an important role in DNA repair in response to DNA damage. (6)(7)(8)(9)(10) In addition, BRCA2 is also involved in the regulation of transcription, (11)(12)(13) cell cycle checkpoints, (14)(15)(16) inhibition of cell growth, (17,18) maintenance of genomic stability, (19)(20)(21)(22) centrosome regulation, (23) and cytokenesis.BRCA2 is specifically phosphorylated in M phase. (10,25,26) Phosphorylation of BRCA2 by the cyclin-dependent kinase cdk1 in M phase blocks C-terminal interaction between BRCA2and RAD51, and reduces the efficiency of recombinational repair.(10)