Since the identification of the human breast and ovarian cancer gene, BRCA1, a large spectrum of germline mutations has been characterized that predispose women to developing these diseases. We have determined the complete coding sequence for the rat BRCA1 homolog and compared it with those of the mouse, dog, and human to help identify the important functional domains of the BRCA1 protein. The overall rat Brcal amino acid identity compared with the predicted mouse, dog, and human gene products is 81%, 69%, and 58%, respectively. In spite of this low overall homology, the amino terminal RING finger domain and one of two nuclear localization signals are highly conserved among these species. In addition, two BRCT domains at the carboxy terminus and a highly acidic region are relatively well conserved. We have also identified several putative regulatory elements through comparison of the bidirectional BRCA1 promoter regions among the rat, mouse, and human genes. These include motifs for CCAAT and G/C boxes, as well as potential SP1, CREB, and NFkB transcription factor binding sites. Finally, analysis of splice variants from rat mammary gland, ovary, testis, spleen, and liver tissues revealed that, while alternative transcripts are detectable, full-length transcripts are the predominant steady-state form.