Women carrying germline mutations in the BRCA1 tumor suppressor gene have a lifetime risk of breast cancer that may be as high as 80 to 90%. Recently, significant progress has been made in understanding the function of the BRCA1 protein through study of its expression patterns, cellular localization, molecular interactions, and potential roles in transcriptional activation and DNA damage response. Despite these advances, fundamental questions regarding the role of BRCA1 in mammary epithelial cell biology remain unanswered. This gap in understanding is especially important since breast cancer is the predominant phenotype in individuals inheriting a mutant BRCA1 allele. As cancer is generally thought to be a disease of dysregulated cellular proliferation and differentiation, we hypothesize that BRCA1 plays specific roles in these processes in mammary epithelial cells. We propose to elucidate the mechanisms by which BRCA1 impacts on proliferation and differentiation in the mammary epithelium of the mouse by addressing the following specific aims: I. Develop and characterize a transgenic mouse system that allows inducible expression of transgenes in a mammary-specific manner. A transgenic mouse system will be generated to permit conditional mammary epithelial cell-specific transgene expression, quantitative control of transgene expression levels, and minimal transgene expression in the uninduced state. II. Use mammary-specific, inducible transgenic system to alter BRCA1 expression in transgenic mice. The inducible system described in aim I will be utilized to perturb BRCA1 expression in vivo. Downregulation of BRCA1 expression will be mediated by conditional expression of a hammerhead ribozyme directed against the endogenous Brca1 transcript. BRCA1 expression will be upregulated via conditional expression of an exogenous BRCA1 minigene. III. Determine the role of BRCA1 in mammary epithelial proliferation in vivo. The impact of dysregulated BRCA1 expression on proliferation in the mammary epithelial cell compartment will be analyzed. In the setting of both upregulated and downregulated BRCA1 expression, proliferation rates in the mammary epithelium will be determined by BrdU labeling. The effects of dysregulated BRCA1 expression on ductal morphology and expression of cell cycle molecules will be determined. IV. Determine the role of BRCA1 in mammary epithelial differentiation in vivo. Upon upregulation and downregulation of BRCA1, alterations in the normal differentiation programs in the mammary gland will be determined. This analysis will be performed at the molecular level by determining the temporal pattern of expression of genes whose expression is normally specific for particular stages of differentiation. In addition the effects of altered BRCA1 expression on the morphologic changes characteristic of lobuloalveolar differentiation in pregnancy will be determined.