Although effective treatments for breast cancer predated the identification of causative molecular defects in humans, it is widely hoped that an understanding and/or manipulation of the key genetic events will lead to even more effective therapies or even cures. Powerful methods of positional cloning and gene identification have identified the breast cancer genes, BRCA1 and BRCA2, which together are responsible for the majority of cases of hereditary breast and ovarian cancer. Although the BRCA1 gene is rarely mutated in sporadic breast or ovarian cancer, levels of BRCA1 mRNA and protein are markedly decreased in the majority of sporadic cases of cancer. This suggests that hereditary and sporadic breast cancer share common genetic themes and that treatments aimed at increasing levels of BRCA1 or BRCA2 may be useful for both hereditary and sporadic cancers. We have demonstrated that gene transfer of wild-type BRCA1 inhibits the growth of sporadic breast and ovarian cancer cells and suppresses growth of established breast and ovarian tumor models in nude mice. Mutant BRCA1 genes do not inhibit growth or suppress tumor, providing additional evidence that BRCA1 is a tumor-suppressor gene. Strategies designed to increase BRCA1 expression or development of BRCA1-mimetic agents may be ultimately useful as therapeutic approaches.