Tumour dMMR is associated with Lynch syndrome (LS), a cancer predisposition syndrome caused by constitutional (germline) pathogenic variants affecting MLH1, MSH2, MSH6, or PMS2. Testing for dMMR in colorectal and endometrial cancers is recommended for LS screening so that identified LS patients can benefit from personalised therapy, cancer prevention, and cancer surveillance [5, 6]. This evidence concerns the gene MLH1 and Leigh syndrome.