MutL Homolog 1 (MLH1) is a protein involved in the mismatch-repair pathway. This protein is commonly associated with hereditary non-polyposis colorectal cancer, as the MLH1 gene is frequently mutated in patients with this cancer. Studies have shown MLH1 to be deficient in a high percentage of patients with microsatellite instability, as well as endometrial and ovarian cancers. Use of Anti-MLH1 is optimized when paired in an IHC panel with MSH6, MSH2, and PMS2. Anti-MLH1 is useful in the detection of MLH1 in a number of normal and neoplastic tissues, and for identifying a loss of MLH1 in tumors that are microsatellite-unstable.