The BRCA1 tumor suppressor has been implicated in a remarkably broad spectrum of cellular processes, including RNA transcription, chromatin remodeling, mRNA processing, apoptosis, cell cycle checkpoint control, centrosome amplification, and several distinct pathways of DNA repair. Nevertheless, the biochemical mechanisms by which BRCA1 carries out these functions is not understood and, as a consequence, it is still unclear why inherited mutations in the BRCA1 gene predispose women to breast and ovarian cancer. The major product of BRCA1 is a polypeptide of 1863 amino acids that contains an N-terminal RING domain and two Cterminal BRCT motifs. To study its biological functions, we sought to characterize protein complexes formed by BRCA1 in rive. This led to identification of the BRCA1-associated RING domain (BARD1) polypeptide, a nuclear protein that resembles BRCA1 in that it also features an N-terminal RING domain and two C-terminal BRCT motifs. The in rive interaction between BARD1 and BRCA1 is mediated by their respective RING domains, and it is disrupted by tumor-associated mutations within the RING motif of BRCA1. Moreover, immunoprecipitation analysis reveals that most, if not all, of the cellular pool of BRCA1 polypeptides is bound to BARD1. On the basis of these results, we proposed that the biological functions of BRCA1, including those responsible for tumor suppression, are mediated by the BRCA1/BARD1 heterodimer. Indeed, recent studies have show that the heteromeric BRCA1/BARD1 complex serves in vitro as an especially potent ubiquitin E3 ligase. Therefore, to evaluate the role of the BRCA1/BARD1 complex in tumor suppression, we will 1) define the catalytic properties of the BRCA1/BARD1 heterodimer and identify the cellular substrates of its enzymatic activity; 2) examine cellular control of the BRCA1/BARD1 heterodimer and identify upstream regulatory factors that modulate its activity during cell cycle progression and in response to genotoxic stress; and 3) define the role of BARD1 in the maintenance of genomic stability. These studies should reveal new insights into the basic mechanisms by which BRCA1 normally suppresses tumor development in normal breast and ovarian tissues.