MLH1 and endometrial cancer: Along the same lines, deficiency of the mismatch repair protein MutL protein homolog 1 (MLH1), commonly observed in Lynch syndrome patients, who generally have a predisposition to develop a broad range of solid tumors, especially colorectal and endometrial cancer (14), leads to loss of regulation of exonuclease 1 (EXO1) activity during DNA repair.