MLH1 and mismatch repair cancer syndrome 1: MMR-deficiency can be caused by Lynch syndrome, resulting from germline pathogenic variants in the DNA MMR genes MLH1, MSH2, MSH6 and PMS2, but other causes include somatic MLH1 gene promoter hypermethylation, constitutional MLH1 gene promoter hypermethylation (MLH1 epimutation), or biallelic somatic MMR gene mutations.