MRC1 and neoplasm: Investigating MMR gene mutations and methylation status in MSI tumor samples, we only observed few cases of MSH6 (MutSα), MSH3 (MutSβ), PMS2 (MutLα), PMS1 (MutLβ), and MLH3 (MutLγ) high-impact mutations often in combination with inactivation of other MMR genes (Supplemental Material; Supplemental Table S3).