Breast and ovarian cancers are the most common diseases affecting women, making the early detection of these fatal diseases a high priority in their medical management. Germline mutations in 17q21 BRCA1, breast cancer susceptibility gene, appear to account for approximately half of familial breast cancers and essentially all families with 17q21-linked inherited susceptibility to ovarian and breast cancer. A second locus, BRCA2 mapped to chromosome arm 13q, appears to account for a proportion of early-onset breast cancer roughly equal to that resulting from BRCA1. Unlike BRCA1, however, BRCA2 may not influence ovarian cancer risk. Like many other genes involved in familial cancer, BRCA1 appears to encode a tumor suppressor, a protein that acts as a negative regulator of tumor growth. Cancer-predisposing alleles typically carry mutations that cause loss or reduction of gene function. The BRCA1 protein (210 kD and alternatively spliced/processed variants of 185, 160, 135 and 85 kD) is a nuclear protein in cultured cells and normal tissues, although a cytoplasmic localization is also reported. It includes an N-terminal RING domain, a negatively charged region in its C-terminal, and a C-terminal acidic domain, partially homologous to yeast RAD9 and to a cloned p53 binding protein. The developmental pattern of murine BRCA1 expression and its cell cycle-regulated expression, suggest a relationship between BRCA1 function and cellular proliferation. The abundance and intracellular location of BRCA1 varies with the cell cycle; BRCA1 protein levels are low in G1, when it is detected as a diffuse immunofluorescent staining throughout the nucleus, and reach a maximum during S phase, when it is localized in the discrete nuclear domains. BRCA1 undergoes cell cycledependent phosphorylation by various cyclin/CDK proteins. BRCA1 Associated Protein 1 (BAP1), a 81 kD protein encoded by the BAP1 gene that is proposed to be a tumor suppressor gene, functions in the BRCA1 growth control pathway, by enhancing the BRCA1-mediated inhibition of breast cancer cell growth. BAP1 is a nuclear-localized, ubiquitin C-terminal hydrolase, that binds to the wild-type RING finger domain of BRCA1. It does not bind to germline mutants of the BRCA1-RING finger found in breast cancer kindreds. Murine BAP1 and BRCA1 are temporally and spatially co-expressed during murine breast development and remodeling, and show overlapping patterns of subnuclear distribution. BAP1 resides on human chromosome 3p21.3, and rearrangements, deletions and missense mutations of BAP1 have been found in lung carcinoma cell lines and in primary breast tumor samples. Antibodies reacting specifically with BAP1 are useful tools in the study of the detailed mechanisms of BRCA1 growth control pathways, and its essential roles during developmental and pathological processes.