MRC1 and hyperinsulinemic hypoglycemia, familial, 4: Many of these cases (7/15 POLE-mutated MSI-H tumors and 5/8 POLD1-mutated MSI-H tumors) were found to have epigenetic silencing of MLH1, the most common mechanism that causes loss of MMR in tumors, suggesting that the high MS indel rates observed in these tumors were indeed due to MMR deficiency.