Breast cancer has often been reported to run in families, and the most important risk factor for the disease is a family history of breast cancer. Numerous pedigrees and segregation analyses have suggested an autosomal dominant transmitted susceptibility to breast cancer. Familial breast cancer occurs alone or associated with other cancers in clinically distinguishable syndromes. Such cases may be characterized by early onset, bilateral disease, prolonged survival, and anticipation, mainly seen as a higher penetrance or earlier onset in subsequent generations. Studies of patients and tumors from these families as well as sporadic cases have led to localization and/or identification of a number of genes implicated in breast carcinogenesis of familial and sporadic breast cancer.