Missense mutations that disrupt the RING domain of the tumor suppressor gene 
BRCA1 lead to increased risk of breast and ovarian cancer. The BRCA1 RING domain 
is a ubiquitin ligase, whose structure and function rely critically on forming a 
heterodimer with BARD1, which also harbors a RING domain. The function of the 
BARD1 RING domain is unknown. In families severely affected with breast cancer, 
we identified inherited BARD1 missense mutations Cys53Trp, Cys71Tyr, and 
Cys83Arg that alter three zinc-binding residues of the BARD1 RING domain. Each 
of these mutant BARD1 proteins retained the ability to form heterodimeric 
complexes with BRCA1 to make an active ubiquitin ligase, but the mutant 
BRCA1/BARD1 complexes were deficient in binding to nucleosomes and in 
ubiquitylating histone H2A. The BARD1 mutations also caused loss of 
transcriptional repression of BRCA1-regulated estrogen metabolism genes CYP1A1 
and CYP3A4; breast epithelial cells edited to create heterozygous loss of BARD1 
showed significantly higher expression of CYP1A1 and CYP3A4 Reintroduction of 
wild-type BARD1 into these cells restored CYP1A1 and CYP3A4 transcription to 
normal levels, but introduction of the cancer-predisposing BARD1 RING mutants 
failed to do so. These results indicate that an intact BARD1 RING domain is 
critical to BRCA1/BARD1 binding to nucleosomes and hence to ubiquitylation of 
histone H2A and also critical to transcriptional repression of BRCA1-regulated 
genes active in estrogen metabolism.