An individual woman's susceptibility to breast cancer is determined by her inherited genetic susceptibility and her personal environment as well as interactions between and within genes and environmental factors. The environment is also complex, consisting of both endogenous and exogenous factors. Importantly, in breast cancer, the susceptibility to environmental agents is highly dependent on the physiological and developmental status of the breast at the time of exposure, i.e., windows of susceptibility. Several major hormonal landmarks in a woman's life impact on the breast's developmental status and modulate susceptibility. This proposal will focus on three post-natal windows of breast cancer susceptibility using both rat and human studies. In rats, these windows include the immature window (3 week old rats) corresponding to childhood in humans, the adolescent window (7 week old rats), and the novel older adult window (65 week old rats) corresponding to peri- menopause in women. It is hypothesized that susceptibility to environmental induced breast cancer is modulated by window-specific risk alleles/genes. Results obtained from these rat models will be extended to women using a comparative genomics approach focusing on genetic epidemiological investigations. Aim 1 will determine if seven previously identified mammary carcinoma susceptibility (Mcs) alleles that have been shown to modulate mammary cancer risk at the adolescent window are unique for this window or will also function at the immature or older adult windows. Aim 2a will phenotype mammary gland at all three windows by integrating gland morphology with global gene expression. Aims 2b and 2c will ask if mammary carcinomas induced by exposure at different windows show window-specific patterns of gene expression and pathway signaling. These expression patterns will be compared to human breast cancer gene expression patterns. Aim 3 will extend work on previous aims by genetically identifying unique quantitative risk loci (QTL) that are not active at the adolescent window but modulate risk at the other two windows. Aim 4 will focus on a comparative genomics/molecular epidemiologic case-control study of over 7,000 women to identify established and suspected GWAS SNPs in human regions homologous to our loci (Aim 1) and QTL (Aim 3). Importantly, all major study planning will include input from partners affiliated with several advocacy and community groups within Wisconsin (Aim 5). Advocates and scientists will jointly communicate important results from these studies as well as studies from other BCERP collaborators.