MRC1 and hereditary nonpolyposis colon cancer: MMR IHC and MSI testing have long been used for colorectal and endometrial carcinomas as a screening test for inherited deleterious alterations in MMR genes, which results in Lynch syndrome, previously called hereditary nonpolyposis colorectal cancer (HNPCC), and accounts for approximately 5% of colorectal carcinomas (CRCs).12 Patients with Lynch syndrome are also at increased risk for neoplasms of the endometrium, upper gastrointestinal tract, pancreaticobiliary system, urinary tract, prostate, ovaries, and brain.12