First from specimens already collected under IRB Protocol #99-030, entitled "Collection of Tissue, Blood, and Cells to be Used for Studying the Causes, Prevention, Diagnosis, and Treatment of Breast Cancer (T. King, PI). Controls will also be collected from patients who have previously undergone genetic counseling through the Clinical Genetics Service at MSKCC and tested negative for germline mutations in BRCA1. Isolation of total RNA from blood will be performed using the PAXgene™ Blood RNA System. This study will use lymphocyte RNA samples from 50 individuals with BRCA1 mutations and 50 age-matched controls. Gene expression profiling will be performed in the Genomics Core Laboratory of MSKCC under the supervision of Agnes Viale, PhD. Class comparison analysis will be performed on the gene expression data in the Biostatistics Department under the supervision of Adam Olshen, PhD. It is anticipated that this study will be completed in approximately 1 year.