Hereditary nonpolyposis colorectal cancer (HNPCC) (Lynch syndrome) accounts for a small proportion of the total colorectal cancer burden, yet represents the most common form of dominantly inherited colon cancer. Until recently, the diagnosis has been based on family history of colorectal and other intra-abdominal cancers. This has been problematic since chance clustering of such tumors cannot be excluded. On the other hand, not every HNPCC patients shows a dramatic family history of cancer. Genetic mapping of a locus for HNPCC to chromosome 2p and the observation that HNPCC tumors show instability of short tandem repeat sequences (replication errors, RER) rapidly led to the cloning of the predisposing gene, human MSH2 (hMSH2). Mutations of hMSH2 have been demonstrated to segregate in large HNPCC families with the cancer phenotype, thus providing convincing evidence that the gene indeed, when mutated, predisposes its carriers to colorectal and other intra-abdominal tumors. Localization of a second locus for HNPCC to chromosome 3p and the subsequent cloning of another predisposing gene, human MLH1 (hMLH1) give hope that a great majority of families can soon be diagnosed by molecular genetic methods.