MLH1 and colorectal cancer: Microsatellite instability (MSI) is the molecular fingerprint of a deficient mismatch repair system. Approximately 15% of colorectal cancers (CRCs) display MSI owing either to epigenetic silencing of MLH1 or approximately 2%-3% of all CRCs which are caused by germline mutations in one of the mismatch repair genes (MLH1, MSH2, MSH6, and PMS2).