MLH1 and Leigh syndrome: Constitutional (mostly germline heterozygous) pathogenic variants in one of the four DNA mismatch repair (MMR) genes (MSH2, MLH1, MSH6 and PMS2) cause Lynch syndrome (LS), although, rarely, LS may be caused by certain mutations of EPCAM (immediately adjacent to MSH2) that also inactivate MSH2 (Poulogiannis et al., 2010; Bellizzi and Frankel, 2009; Frankel et al., 2019).