Further confirmation of its role in breast tumors was the discovery that the MCF-7 breast cancer cell line contains a RAD51C-ATXN7 fusion gene consisting of RAD51C exons 1�C7 and ATXN7 exons 6�C13. Other RAD51 gene family members had been associated with increased risk of breast cancer, but there had been no reports implicating RAD51C. To further investigate the role of RAD51C as an HBOC predisposition gene, we performed complete sequencing of RAD51C to screen for mutations in 286 BRCA1and BRCA2negative breast and/or ovarian cancer cases with a family history of breast and ovarian cancer. Fifteen variants were identified, of which we selected four non-synonymous variants for functional studies; RAD51C-G153D and RAD51C-T287A because they alter highly conserved amino acid residues, RAD51C-A126T because it alters an amino acid in the Walker A box, and RAD51C-R214C because it alters an amino acid that separates a beta-strand from an alphahelix. Yeast two-hybrid and immunoblot assays indicated that the RAD51C-T287A and RAD51C-A126T mutations do not substantially alter either the ability of RAD51C to interact with its companion proteins or its steady-state level, consistent with the results of the functional assays of Meindl at al.. Furthermore, these two variants have been identified multiple times in other studies, suggesting that they are benign polymorphisms. However, the results of the yeast two-hybrid and immunoblot assays were consistent with RAD51C-G153D being a pathogenic mutation as it led to undetectable levels of interaction between RAD51C and both XRCC3 and RAD51B, but did not markedly change levels of RAD51C. We hypothesize that the RAD51C-G153D mutation critically alters the ability of RAD51C to interact with XRCC3 and RAD51B, although it is not in a region of known secondary structure. RAD51C-R214C may be a hypomorphic mutation of uncertain clinical significance. Small but significant reductions in interaction between RAD51C and both XRCC3 and RAD51B were observed, but there were no marked changes in the steady-state level of RAD51C. Unfortunately, there were no additional samples in the families in order to investigate cosegregation with breast and ovarian cancer or tumor tissue to determine if there was loss of heterozygosity of the wild-type allele. They identified 14 monoallelic germline mutations of which 6 were considered pathogenic. The mutations were only identified in cases from families with both breast and ovarian cancers and in no cases from families with only breast cancer . In a Spanish study of 492 BRCA1and BRCA2 tested-negative breast cancer patients with family history of breast and/or ovarian cancer, they identified 12 variants, of which one was clearly pathogenic in the subset of 106 cases with a family history of both breast and ovarian cancers. Interestingly, the one case was of micturition bladder Swedish origin, and this mutation was recently reported in an ovarian case in a study of Swedish and Finnish familial breast cancer cases and unselected ovarian cancer cases. That study also reported a second clearly pathogenic mutation in an HBOC breast cancer case. In a recent Finnish study, two recurrent deleterious mutations were identified, and specifically in those with a personal or family history of ovarian cancer. In a Chinese study of 275 women from HBOC families, two possibly pathogenic mutations were found. In two additional studies of 454 and 92 breast and/or ovarian cancer cases from HBOC families, no pathogenic mutations were found.