PDCD1 and colorectal carcinoma: Although remarkable advances have been made in cancer immunotherapy in recent years and some immunotherapy targets such as PD-1, PD-L1 and CTLA4 have been clinically applied [7], it benefits only 3% to 7% of CRC patients who exhibit defective mismatch repair (dMMR) proteins/microsatellite instability (MSI-H) [8,9].