Germline alterations of BRCA1 result in susceptibility to breast and ovarian cancer. The protein encoded by BRCA1 interacts in vivo with the BRCA1-associated RING domain (BARD1) protein. Accordingly, BARD1 is likely to be a critical factor in BRCA1-mediated tumor suppression and may also serve as a target for tumorigenic lesions in some human cancers. We have now determined the genomic structure of BARD1 and performed a mutational analysis of 58 ovarian tumors, 50 breast tumors and 60 uterine tumors. Seven polymorphisms were detected within the 2.34 kb coding sequence of BARD1 . Somatically acquired missense mutations were observed in one breast carcinoma and one endometrial tumor; in at least one of these cases, tumor formation was accompanied by loss of the wild-type BARD1 allele, following the paradigm for known tumor suppressor genes. In addition, a germline alteration of BARD1 was identified in a clear cell ovarian tumor (Gln564His); again, loss of the wild-type BARD1 allele was observed in the malignant cells of this patient. The Gln564His patient was also diagnosed with two other primary cancers: a synchronous lobular breast carcinoma and a stage IA clear cell endometrioid cancer confined to an endometrial polyp 6 years earlier. These findings suggest an occasional role for BARD1 mutations in the development of sporadic and hereditary tumors.