MRC1 and colorectal carcinoma: Implementation of Jerusalem guidelines, which recommend conducting MMR proteins immunohistochemistry (IHC) in all CRC or endometrial cancers in patients younger than 70 years old [3] or universal screening, has increased the number of patients diagnosed with LS but has also revealed a large number of patients (~30%) who present with tumor microsatellite instability (MSI) or loss-of-expression MMR proteins but without evidence of germline pathogenic mutation in these genes [4].