Along these lines, there is broad consensus in that the subset of patients with a clinical response to PD-1 therapy consist of those harbouring a tumour microsatellite instability-high (MSI-H) phenotype, also called deficient DNA mismatch repair (dMMR) CRC, in contrast to proficient mismatch repair (pMMR) CRC [20], with MLH1, MSH2, MSH6 and PMS2 as the main MMR gene products. Here, MLH1 is linked to colorectal carcinoma.