MMR deficiency (dMMR) occurs in ∼10%–15% of all CRCs and 3% of these are associated with LS—an autosomal dominant disorder caused by germline mutations in MMR genes (such as MLH1, MSH2, MSH6, or PMS2); the other 12% are sporadic CRC, caused by hypermethylation of the MLH1 promoter [4, 5]. The gene discussed is MLH1; the disease is Leigh syndrome.