This disease of breast cancer (BrCa) has been intensively studied over the past quarter century, yet major gaps remain in our understanding of causal mechanisms operating at the level of molecules, cells and tissues; these shortfalls continue to compromise our ability to predict malignant progression of primary BrCa's and identify novel targets for clinical intervention. This Project involves an interacting consortium o BrCa researchers who intend to uncover causative mechanisms operating at a succession of steps of tumor development. One project examines how defects in the BRCA1 protein generate the mutations that create mutant BrCa cell genomes in genetically predisposed patients and in a portion of sporadic tumors. A second examines how the reproductive history of women interacts with the differentiation of distinct cell types to increase or reduce BrCa formation. A third studis how the hormonal environment of a woman together with the actions of the BRCA1 gene lead to changes in gene regulation, the outgrowth of these tumors, and the acquisition of resistance to existing therapies.. A fourth examines how signals released by oncoproteins affect the cell-cycle apparatus and how these signaling channels offer a striking opportunity for therapeutic intervention. A fifth will determine whether distinct subpopulations of carcinoma cells within primary BrCa's collaborate with one another in enabling tumor growth, while a six examines how signals released by the BrCa-associated stroma induce primary BrCa cells to acquire traits of highly malignant cells that enable them to invade and metastasize. This last program becomes highly relevant to reducing BrCa-associated mortality, since the vast majority of deaths derive from the metastases generated by these tumors rather than from primary growths within the breast. Together, the proposed Projects hold the promise of elucidating key steps in tumor formation that have offered or will offer novel molecular targets for therapeutic intervention.