Since BRCA1 was cloned a decade ago, significant progress has been made in defining its biochemical and biological functions, as well as its role in breast and ovarian cancers. BRCA1 has been implicated in many cellular processes, including DNA repair, cell cycle checkpoint control, protein ubiquitination and chromatin remodeling. This review examines the role(s) of BRCA1 in mediating these cellular processes, and discusses its potential involvement in the resistance of breast cancer to radiation-based therapies. Finally, the possibility that BRCA1-associated proteins may serve as new targets for breast cancer radiation therapy is explored. The activation or inactivation of these BRCA1-associated proteins may modify both the risk of developing cancers in BRCA1 mutation carriers and the efficacy of breast cancer therapy, including radiation.