The immunohistochemical detection of either an abnormal expression or a loss of expression of the mismatch repair proteins, MLH1, MSH2, MSH6, and PMS2, is of significant diagnostic and prognostic value in CRC, as well as for the detection of hereditary nonpolyposis colorectal cancer (HNPCC), also known as Lynch syndrome, which constitutes approximately 2% to 3% of all colorectal carcinomas [19,20,21]. The gene discussed is PMS2; the disease is hereditary nonpolyposis colon cancer.