Following the cloning of RB1, more than 40 hereditary cancer genes have been cloned, including the gene for familial adenomatous polyposis, APC. As with retinoblastoma, APC was shown to be somatically mutant in a large majority of nonhereditary colon cancers, thereby stimulating interest in the study of hereditary cancer for the purpose of illuminating the biology of all cancer. This evidence concerns the gene APC and retinoblastoma.