PDCD1 and colorectal cancer: Although programmed cell death-ligand 1 (PD-L1) and programmed cell death 1 (PD1) inhibitors have been approved for the treatment of advanced colorectal cancer (CRC) with a deficiency in mismatch repair (dMMR) and/or high microsatellite instability (MSI), patients with proficient mismatch repair (pMMR) and microsatellite stable (pMMR/MSS) CRC, accounting for more than 80% of all CRC cases, exhibit poor responses to immune checkpoint inhibitors [1,2].