MLH1 is a mismatch repair gene of around 87 kDa, commonly associated with Hereditary Non-Polyposis Colorectal Cancer (HNPCC). This gene was identified as a locus frequently mutated in HNPCC. It is a human homolog of the E. coli DNA mismatch repair gene mutL, consistent with the characteristic alterations in microsatellite sequences (RER+ phenotype) found in HNPCC. Alternatively, spliced transcript variants encoding different isoforms have been described, but their fulllength natures have not been determined. In a high proportion of patients with microsatellite instability (MSI-H), the MLH1 protein is typically deficient. This protein deficiency is linked to the autosomal dominant condition of Hereditary Non-Polyposis Colon Cancer. The anti-MLH1antibody is useful in screening patients and families for this condition. Colon cancers that are microsatellite-unstable have a better prognosis than their microsatellite stable counterparts.^1-6