"I was attracted to studies of cancer families because epidemiological studies show that virtually all cancers manifest a tendency to aggregate in families. Close relatives of a cancer patient are at increased risk of that neoplasm, and perhaps other forms of cancer. The excess site-specific cancer risk is exceptionally high for carriers of certain cancer genes, in whom the attack rate can approach 100 percent. In candidate cancer families, the possibility that clustering is on the basis of chance must be excluded through epidemiological studies that establish the presence of an excess cancer risk. Predisposed families are candidates for laboratory studies to identify the inherited susceptibility factors. These investigations have led to the identification and isolation of human cancer genes, the tumor suppressor genes. These cancer genes are among more than 200 single-gene traits associated with the development of cancer. Approximately a dozen inherited susceptibility genes have been definitively identified, and many more are being sought. From studies of retinoblastoma and other rare cancers, important new information was generated about the fundamental biology of cancers that arise in many patients. Isolation of an inherited cancer susceptibility gene provides opportunities for presymptomatic testing of at-risk relatives. However, testing of healthy individuals also raise important issues regarding informed consent, confidentiality and potential for adverse psychological, social and economic effects."