Breast cancer occurs in hereditary and sporadic form. Breast cancer susceptibility gene (BRCA1) is known to be responsible for hereditary breast cancer. BRCA1 mutations are rare in sporadic cancers, but loss of BRCA1 resulting from reduced expression or incorrect subcellular localization is postulated to be important in non-familial breast cancers. More than 300 germline mutations have been identified so far in patients with familial breast and/or ovarian cancer, however, only a few somatic mutations have been identified in sporadic breast cancer. The decreased expression of BRCA1 in sporadic breast cancer is thought to be caused by other mechanisms, such as CpG methylation. BRCA1 expression may play an important role in the pathogenesis and prognosis of sporadic breast carcinoma.