MLH1 and hereditary nonpolyposis colon cancer: Tumor MMR status can also be determined by immunohistochemical (IHC) analysis of the protein products of MLH1, MSH2, MSH6 and PMS2. While the gold standard for HNPCC diagnosis is genomic sequencing, screening strategies testing for loss of protein expression by IHC versus polymerase chain reaction detection of the MSI panel are considered to be equivalent [4, 5].