BRCA1 was the first breast cancer susceptibility gene to be identified and cloned. In individuals from high-risk families, mutations in BRCA1 increase the lifetime risk of developing breast cancer eight to tenfold, compared to the general population. How the BRCA1 protein product normally functions to suppress tumor formation and how defects in the gene can ultimately lead to breast cancer have been the focus of intense scrutiny by the scientific and medical communities. BRCA1 has intrinsic transactivation activity and is able to activate the p21 promoter. In addition, BRCA1 is linked to a number of genes involved in transcriptional regulation, including CtIP, c-Myc, the RNA holoenzyme complex, and the histone deacetylase complex. Moreover, BRCA1 is essential for cellular response to DNA damage repair. Inactivation of Brca1 in mouse embryonic stem and fibroblast cells results in increased cell sensitivity to DNA-damaging agents. In human cells, BRCA1 binds to both Rad50 and Rad51 and colocalizes with these proteins at repair foci. Part of BRCA1's response to DNA damage may in fact be corroborated through transcriptional regulation. The expression of GADD45, a DNA damage-responsive gene, is increased immediately after induction of BRCA1. Recently, BRCA1 was shown to repress estradiol (E2)-responsive ER-β£mediated transcriptional activity, potentially linking the multiple functions of BRCA1 to specific tissue targets. These recent developments in BRCA1 function are an encouraging step toward understanding the role of BRCA1 in breast cancer formation.