LS is caused either by germline loss-of-function (LoF) pathogenic (P) and likely pathogenic (LP) variants in one of the four DNA mismatch repair (MMR) genes (MLH1, MSH2, MSH6, and PMS2) or by germline deletions in the epithelial cell adhesion molecule (EPCAM) gene leading to epigenetic silencing of the adjacent MSH21,2. The gene discussed is MLH1; the disease is Leigh syndrome.