Germ line mutation of MMR genes, particularly MLH1 or MSH2, is the basis of hereditary non-polyposis colorectal cancer (HNPCC), or Lynch syndrome, which confers susceptibility to colorectal cancer but also to other specific cancer types including endometrial and ovarian cancer (reviewed in 3,4). This evidence concerns the gene MSH2 and hereditary nonpolyposis colon cancer.