An estimated 5 to 10% of all breast and ovarian cancer is attributable to inherited mutations in two highly penetrant autosomal dominant susceptibility genes, BRCA1 and BRCA2. BRCA1 confers higher risk of ovarian cancer and BRCA2 much higher risk of male breast cancer. With the exception of missense mutations in the RING finger near the amino terminus of BRCA1, virtually all germline mutations in the gene cause the novel BRCA1 protein to be prematurely truncated. Approximately 90% of breast tumors in BRCA1 families, 50% of unselected breast tumors and 65-80% of unselected ovarian tumors have lost one allele of BRCA1 by somatic deletion. Very few tumors have detectable somatic point mutations in BRCA1. Inhibition of BRCA1 expression in mammary epithelial cell lines also suggests that BRCA1 may act as a tumor suppressor. The biological function of BRCA1 is still unknown, although identification of a patient homozygous for an inherited BRCA1 mutation suggests that the gene's function may be essential only to specific tissues. At least two other genes, P53 and the androgen receptor, are responsible for inherited predisposition to breast cancer in rare families. Several epidemiologic studies suggest that individuals carrying rare alleles at a minisatellite flanking the HRAS locus are at increased risk of cancer, including breast cancer. Finally, preliminary epidemiologic studies also suggest that individuals heterozygous for mutations in the ataxia telangiectasia gene may be at increased risk of breast cancer.