Abstract: Breast ovarian cancer susceptibility (BRCA) proteins appear to be involved in cell cycle regulation, DNA repair or genome integrity and induction of apoptosis in a variety of cells from humans and laboratory animals. The BRCA gene and protein have not been identified in cattle. The pattern of BRCA1 gene expression during normal mammary gland development and involution has not been examined in detail in any mammalian species. Therefore, the purpose of the present study was to clone the BRCA1 gene in Holstein dairy cattle and determine if the BRCA1 gene is differentially expressed through various stages of mammary gland development. We also localized immunoreactive BRCA1 protein in bovine mammary cells and milk fat globule membrane. Bovine BRCA1 cDNA was highly conserved to the human. Five hundred base pairs of exon 11 (+3384/ + 3866, human BRCA1 cDNA position) and the C-terminus 1 kb were identical to the human. Seven hundred base pairs of the N-terminus, which contain two ring domains, showed 90 % homology to human BRCA1. In bovine tissues, the degree of BRCA1 gene expression, from highest to lowest , was as follows: liver, spleen, mammary tissues and kidney. The mammary tissues of early pregnancy heifers (3 months) showed much higher mammary gland BRCA1 mRNA expression than other mammary developmental stages. BRCA1 expression declined during the remainder of pregnancy and remained low for lactating cows. However, its expression increased when involution proceeded. Immunohistochemical studies showed that immunoreactive BRCA1 was localized in the nucleus and cytoplasm of mammary epithelial cells from lactating cows. It was not present in myoepithelial cells. The protein was also localized in the milk fat globule membranes. Our data suggest that BRCA1 is involved in bovine mammary gland development and/or differentiation, is specifically localized in secretary epithelial cells and is likely a secreted protein during normal lactation.