BRCA1, a breast and ovarian cancer susceptibility gene, encodes a 22-kDa protein. BRCA1 contains an N-terminal RING finger that mediates protein-protein interaction. The C-terminal domain of BRCA1 (BRCT) can activate transcription and interacts with RNA polymerase holoenzyme. BRCA1 in transcriptional activation of ATF1 target genes, some of which are involved in the transcriptional response to DNA damage. The BRCA1 gene encodes a tumor suppressor that is mutated in 5% of familial breast cancers. The BRCA1 protein has been implicated in the DNA damage response, as DNA damage induces the phosphorylation of BRCA1 and causes its recruitment into nuclear foci that contain DNA repair proteins. BRCA1 is involved in the regulation of multiple nuclear events including transcription. AD1, one of the two trans-activation domains in BRCA1, stimulates transcription in a cell context-dependent manner. The coiled-coil-mediated cooperation between BRCA1 and JunB may facilitate the function of these proteins in tissue-specific transcriptional regulation and tumor suppression. Germline mutations in the breast cancer susceptibility genes, BRCA1 and BRCA2, are thought to account for a large portion of familial breast cancer. BRCA1 and BRCA2 participate in cell cycle progression, apoptosis, and DNA repair pathways.