MLH1 and Leigh syndrome: Lynch syndrome (LS), also known as hereditary nonpolyposis colorectal cancer syndrome, is an autosomal dominant genetic disease caused by mutations in DNA mismatch repair (MMR) genes.[1,2] The role of MMR includes three aspects: maintaining the fidelity of DNA during the process of replication, reducing the occurrence of microdeletion and microinsertion caused by the decline in DNA polymerase or missense mutation during the process of replication and folding, and maintaining the stability of DNA.[3,4] MMR genes mainly include MLH1, MSH2, MSH6, and PMS2.