Heredity of malignant diseases has been an inexplicable issue for a long time. Because there was less precise description or undeveloped genetic science, most of those cases were explained as a coincidental accumulation of cancer. Recent advantage of microbiology revealed many of responsible genes for diseases including some malignancies. Familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC) are well-investigated autosomal dominant diseases among them. In 1992, HNPCC patients in Japan were firstly screened by newly developed Japanese Clinical Criteria. During the study, 777 families were collected from 32470 colorectal cancer patients in 60 institutions in Japan. They showed features as an early age of onset, right colon involvement, and association of many other malignancies. Currently, hereditary tumor syndromes are classified in two categories ; A) Hereditary tumor syndrome including FAP, HNPCC, multiple endocline neoplasia (MEN) I and II etc, and B) Hereditary condition susceptible for cancer including xeroderma pigmentosusm, ataxia pigmentosum etc. Present investigations suggest an incidence of hereditary cancers as high as 5-10% of all cancers. It is not so high but study of these conditions may give a great advantage upon a carcinogenesis and treatment of all other malignancies. However, we have to pay a careful attention to the ethical, legal, and social issue that might be developed by the study
