Hereditary cases of breast and ovarian cancer are often attributed to germ-line 
mutations of the BRCA1 tumor suppressor gene. Although BRCA1 is involved in 
diverse cellular processes, its role in the maintenance of genomic integrity may 
be a key component of its tumor suppression activity. The protein encoded by 
BRCA1 interacts in vivo with the related BARD1 protein to form a heterodimeric 
complex that acts as a ubiquitin E3 ligase. Because the enzymatic activity of 
the BRCA1/BARD1 heterodimer is conserved over a broad phylogenetic range, it is 
thought to be critical for the central functions of BRCA1. To test this 
hypothesis, we have generated isogenic clones of embryonic stem cells that do or 
do not express an enzymatically proficient Brca1 polypeptide. Surprisingly, 
cells lacking the ubiquitin ligase activity of BRCA1 are viable and do not 
accumulate spontaneous cytogenetic rearrangements. Gene targeting efficiencies 
are modestly reduced in these cells, and chromosomal rearrangements arise at 
elevated rates in response to genotoxic stress. Nonetheless, cells lacking Brca1 
enzymatic activity are not hypersensitive to the DNA cross-linking agent 
mitomycin C. They also form Rad51 focus in response to ionizing radiation and 
repair chromosome breaks by homologous recombination at wild-type levels. These 
results indicate that key aspects of BRCA1 function in genome maintenance, 
including its role in homology-directed repair of double-strand DNA breaks, do 
not depend on the E3 ligase activity of BRCA1.