DNA mismatch repair protein Mlh1 or MutL protein homolog 1 is a protein that in humans is encoded by the MLH1 gene located on chromosome 3. It is a gene commonly associated with hereditary nonpolyposis colorectal cancer. MLH1 protein is one component of a system of seven DNA mismatch repair proteins that work coordinately in sequential steps to initiate repair of DNA mismatches in humans [PMID: 18543306]. Defects in mismatch repair, found in about 13% of colorectal cancers, are much more frequently due to deficiency of MLH1 than deficiencies of other DNA mismatch repair proteins [PMID: 15887099]. The seven DNA mismatch repair proteins in humans are MLH1, MLH3, MSH2, MSH3, MSH6, PMS1 and PMS2.