The human genes BRCA1, conferring susceptibility to early-onset breast and ovarian cancer, has recently been isolated. Here we describe isolation of cDNAs, sequence analysis, and genomic localization of the murine homolog, Brac1. The mouse cDNA sequence predicts a protein of 1812 amino acids; a number of small gaps account for the 51 fewer residues in the mouse protein relative to human BRCA1. While the predicted mouse and human proteins display on the whole a high level of homology (58% identity, 73% similarity), the regions of greatest homology are at the respective amino and carboxyl termini. Most reported disease-associated missense mutations in human BCRA1 occurred within these more highly conserved terminal regions. A predicted zinc-building RING finger domain near the amino terminus lies within a 50 amino acid stretch that is perfectly conserved in both species. The strong conservation during mammalian evolution argues for the importance of this domain, perhaps mediating a role for BRCA1 in DNA and/or protein binding. We have also identified a conserved highly acidic domain in the carboxyl terminal half of the BCRA1 protein resembling acidic transactivation domains of certain transcription factors. Using an interspecific backcross panel, Brca1 was mapped to a region of mouse chromosome 11 that exhibits conserved linkage with 17q21. The sequence and isolated cDNAs will provide useful reagents for studying the expression of Brca1 in the mouse, and for testing the importance of the evolutionarily conserved domains.