This chapter reviews more commonly described genetic changes associated with hereditary and nonhereditary forms of breast and ovarian cancer and their relationship to prognosis. The majority of breast and ovarian cancers are nonhereditary and are not associated with dominantly inherited mutations in genes such as BRCA1 and BRCA2. The chapter summarizes the data available concerning genes for which there is strongest evidence for a role in the development of nonhereditary breast and ovarian cancers. Despite the fact that germline mutations in BRCA1 and BRCA2 occur at high frequency in hereditary forms of breast and ovarian cancers, few mutations have been reported in nonhereditary forms. Many regions of loss of heterozygosity (LOH) have been reported in breast and ovarian tumors, and are suggestive of the location of tumor suppressor genes. The inhibition of apoptosis or programmed cell death plays an important role in cancer development. A large number of genes have been implicated in the regulation of apoptosis.