MLH1 and endometrial cancer: The mismatch repair deficiency endometrial cancer sub-group comprises about 25–30% of all endometrial cancers and is defined by the loss of nuclear expression of one or more mismatch repair proteins, leading to an accumulation of mismatches, insertions, and deletions.39 44 47 Mismatch repair deficiency is most often caused by somatic events such as MLH1 promoter hypermethylation.