MRC1 and Leigh syndrome: LS is caused by germline inactivation of one of the mismatch repair (MMR) genes (MLH1, MSH2, MSH6, or PMS2) or EPCAM gene deletions, and its molecular diagnosis mainly relies on MMR‐deficiency (MMRd) screening in tumors (microsatellite instability [MSI] assessment and/or MMR immunohistochemical [IHC] staining) along with germline MMR testing.2