Cancers can be classified as hereditary or sporadic based on the difference in the involvement of genetic factors. Hereditary cancers, which account for approximately 2% of all cancers, are caused by germline mutations of defined genes inherited in an autosomal dominant or recessive manner. Dozens of tumor suppressor genes, oncogenes and DNA repair genes have been identified as being responsible for hereditary cancers. Common cancers (sporadic cancers) make up the remaining 98% of all cancers. It is considered that susceptibility to sporadic cancers in each individual is determined by the combination of multiple genetic polymorphisms. Case-control studies, mainly focusing on the genes involved in drug metabolisms and DNA repair, have shown that dozens of genes are associated with cancer risks.