MRC1 and neoplasm: The MSI tumor phenotype is caused by a deficiency of the MMR system, resulting from a MMR gene germline mutation (MLH1, MSH2, MSH6, PMS2; i.e., Lynch syndrome) or from an epigenetic inactivation of MLH1 or double somatic mutations in the MMR genes (i.e., sporadic cancers) [1,2].