The specific aim of this project is to test the hypothesis that BRCA1 haploinsufficiency regulates gene expression on the X chromosome. This hypothesis will be examined by determining if X chromosome gene expression profiles, derived from lymphocyte RNA, of BRCA1 mutation carriers (cases) can be distinguished from an age-matched group of women who do not have BRCA1 mutations (controls). Neither cases nor controls will have a history of cancer. This investigation will provide valuable insight into the biologic function of BRCA1 and its role in breast and/or ovarian tumorigenesis. Blood samples from cases will be collected from patients who have previously undergone genetic counseling through the Clinical Genetics Service at MSKCC and tested positive for a germline mutation in BRCA1. Blood samples from controls will be obtained from two sources.