Mendelian hereditary CRC syndromes are mainly due to germline mutations in APC, MUTYH, and the MMR genes (MSH2, MSH6, PMS2, and MLH1), which cause, respectively, familial adenomatous polyposis (FAP; #175100); MUTYH-associated polyposis (MAP; #608456); Lynch syndrome )#120435); or constitutional mismatch repair deficiency (CMMRD; MIM #276300) in the biallelic mode [3,11]. The gene discussed is MRC1; the disease is Lynch syndrome.