The disorder is diagnosed by molecular or immunohistochemistry (IHC) testing in patients with mutations in one of four mismatch repair (MMR) genes designated MLH1, PMS2, MSH6 and MSH2. Hereditary CRC is a priority of the Iranian Hereditary Colorectal Cancer Registry (IHCCR) that aims to detect, register and follow these patients. Here, MLH1 is linked to colorectal carcinoma.