MMR deficiency results from either germline mutation of one of the MMR genes (Lynch syndrome, 20% of dMMR/MSI CRCs), or somatic inactivation of one of the MMR genes, mostly MLH1 by hypermethylation of its promoter (sporadic cases, 80% of dMMR/MSI CRCs) [3]. This evidence concerns the gene MRC1 and mismatch repair cancer syndrome 1.