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 The gene discussed is MRC1; the disease is endometrial carcinoma.