Lynch syndrome, formerly known as hereditary nonpolyposis colorectal cancer (HNPCC) [1], is an autosomal dominantly inherited disorder of cancer susceptibility caused by germline mutations in the DNA mismatch repair (MMR) genes, MLH1, MSH2, MSH6 and PMS2. The prevalence of individuals who carry a pathogenic germline mutation in one of these genes in the population is estimated to be, depending on various assumptions, from 1 in 370 to 1 in 3,000 [2-4]. Here, MLH1 is linked to hereditary nonpolyposis colon cancer.