PDCD1 and cancer: In recent years, immune checkpoint inhibitors (ICIs), especially monoclonal antibody targeting immune checkpoints such as programmed cell death protein 1 (PD-1), programmed death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) have transformed the treatment of many cancers (Kennedy and Salama, 2020).