BRCA1 Associated RING Domain gene 1 (BARD1) interacts with the N terminal region of BRCA1. In addition to its ability to bind BRCA1 in vivo and in vitro, BARD1 shares homology with the 2 most conserved regions of BRCA1: the N terminal RING motif and the C terminal BRCT domain. The RING motif is a cysteine rich sequence found in a variety of proteins that regulate cell growth, including the products of tumor suppressor genes and dominant protooncogenes. The BARD1 protein also contains 3 tandem ankyrin repeats. The BARD1/BRCA1 interaction is disrupted by tumorigenic amino acid substitutions in BRCA1, implying that the formation of a stable complex between these proteins may be an essential aspect of BRCA1 tumor suppression. BARD1 may be the target of oncogenic mutations in breast or ovarian cancer. BARD1 also plays a role in mediating apoptotic stress and p53 dependent.