MMR deficiency occurs through mutations in genes that recognize and correct errors in mismatched nucleotides (MLH1, MSH2, MSH6, and PMS2) or through methylation-induced gene silencing of the promoter of MLH1. Germline mutations in MMR proteins are associated with Lynch syndrome, although a majority of MMR deficient tumors are sporadic and occur through MLH1 promoter methylation. The gene discussed is MRC1; the disease is mismatch repair cancer syndrome 1.