One of the principal objectives of our laboratory is to understand the mechanisms by which hormones and growth factors regulate normal mammary gland development and how these same regulatory pathways become altered in breast cancer. Specific interest has been placed upon studying the mechanisms by which the lactogenic hormones, prolactin and glucocorticoids, regulate milk protein gene expression. Composite response elements (CoRE) containing binding sites for C/EBPB, YY-1, STAT5, and GR appear to mediate the hormonal and developmental regulation of a particular milk protein gene, b-casein. It is our hypothesis that lactogenic hormones and local growth factors stimulate the transcription of the B-casein gene through a specific and ordered assembly of transcription factors and comodulatory proteins at the proximal promoter and distal enhancer. The specific aims of this proposal employ molecular biology techniques to delineate the order of assembly of transcription factors and coregulatory proteins at the B-casein promoter and enhancer, and utilize mouse genetics to define the mechanisms by which these proteins regulate ductal morphogenesis and lobuloalveolar development of the mammary gland. This model system provides a unique opportunity to understand how the synergistic interactions of peptide and steroid hormone-regulated signal transduction pathways result in the recruitment of transcription factors and comodulatory proteins at both a proximal promoter and a distal enhancer to control tissue-specific gene expression.