Approximately 10 to 15 percent of all breast cancers are thought to be familial and about one third of these cases are due to an inherited mutation in a BRCA1 or BRCA2 breast cancer-susceptibility gene. The lifetime incidence of breast cancer in mutation carriers is above 50 percent, and carriers of BRCA1 mutation also have a substantially increased risk of ovarian cancer. BRCA1 and 2 mutations are associated with early-onset breast cancer, and some experts call for aggressive screening of affected persons. Monthly self-examination of the breasts beginning at age 18 and annual clinical examinations and mammography after age 25 have been recommended but are of unproven benefit. Prophylactic mastectomy and oophorectomy have been advocated by some authorities, but these interventions are disfiguring and for some carriers of the gene, they are unnecessary. The patient's decision to undergo genetic screening is complicated by the technical difficulty of the test, the substantial cost and the still incomplete understanding of the penetrance of disease in known mutation carriers.