EXO1 and Leigh syndrome: Some cases were characterized by somatic biallelic inactivation, and some are likely to be true LS cases with germline mutations that could not be detected, possibly because of (1) mutations in untested regulatory regions of MMR genes, such as the promoter regions, untranslated regions, or deep intron sequences32; (2) genomic rearrangements or complex mutations that are not readily identified by current sequencing techniques; or (3) direct or indirect involvement of MMR function via germline mutations in other genes, such as EPCAM, MLH3, MSH3, EXO1, PMS1, and TGFBR.