Mutations inactivating key components of the human MMR system (hMLH1, hMSH2, hMSH6, or hPMS2) are responsible for the most common hereditary cancer predisposition syndrome referred to as Lynch Syndrome or hereditary non-polyposis colorectal cancer (HNPCC)5–7. This evidence concerns the gene MSH2 and hereditary nonpolyposis colon cancer.