Lack of expression of MMR proteins by immunohistochemistry (IHC) (primarily using antibodies to the MLH1 and MSH2 proteins) is diagnostic for dMMR and is often used in MSI tumor analysis as an alternative to PCR, as well as to complement genetic testing for HNPCC patients [188,191]. The gene discussed is MRC1; the disease is hereditary nonpolyposis colon cancer.