The MSI-H phenotype results from either a somatic inactivation of MLH1 MMR gene (sporadic cases, 12%) or from a germline mutation in MMR genes (MLH1, MSH2, MSH6, PMS2) leading to a deficient DNA mismatch repair (dMMR); such as in the case of the Lynch syndrome (3%) (14). This evidence concerns the gene MLH1 and Lynch syndrome.