Lynch syndrome (formerly known as hereditary nonpolyposis colorectal cancer or HNPCC) is an autosomal dominant cancer predisposition syndrome with a variable penetrance; it results from the inheritance of a heterozygous mutation affecting one of the key mismatch repair genes: MLH1, MSH2, MSH6, or PMS2 [3–5]. This evidence concerns the gene MLH1 and hereditary nonpolyposis colon cancer.