Heterozygous pathogenic variants in the MMR genes predispose to Lynch syndrome (LS, OMIM #120435), also known as hereditary nonpolyposis colorectal cancer (HNPCC), while homozygous or compound heterozygous pathogenic variants lead to constitutional MMR deficiency (CMMRD, OMIM #276300) syndrome. This evidence concerns the gene MRC1 and Leigh syndrome.