Women who have mutations in one of two breast cancer susceptibility genes, BRCA1 and BRCA2, have up to an 80% risk of developing breast cancer in their lifetimes. Hereditary breast cancers harboring these mutations show unique characteristics indicative of these cancer-driving mutations. Breast cancers with BRCA1 mutations are more aggressive and have a poor prognosis compared to sporadic breast cancers. The goals of this proposal are to identify new potential therapeutic targets for BRCA1 breast cancers and to further our understanding of the mechanisms underlying this type of breast cancer. Target identification is a major obstacle to the development of new breast cancer therapies. An ideal target for breast cancer therapeutic intervention is one that upon inhibition leads to preferential killing of breast cancer cells rather than normal breast cells. Screens that measure an outcome after inhibiting individual genes on a large-scale basis are powerful experiments that could identify such targets, but until recently these screens could not be performed in human cells. To perform these screens, our laboratory has developed a new technology to individually inactivate every gene in the human genome, approximately 23,000 genes, one at a time. This proposal hypothesizes that we can identify new therapeutic targets for breast cancers with BRCA1 mutations using this screening technology to determine which genes, when inactivated, lead to cell death of BRCA1 mutant breast cancer cells but not breast cancer cells with normal BRCA1. In addition to identifying such genes, we also propose to characterize the mechanisms by which BRCA1 breast cancer cells are dependent on these genes. This study proposes the first screen of its kind to identify and characterize genes that are selectively required for BRCA1 mutant breast cancer cell viability. This effort will provide a comprehensive exploration of the dependencies and vulnerabilities specific to BRCA1 mutant breast cancer cells. Importantly, this strategy should uncover many previously unrecognized targets for breast cancer drug discovery and has the potential to provide additional, more effective treatment options for breast cancer patients with BRCA1 mutations.