PDCD1 and cancer: To date, clinically approved cancer immunotherapy includes immune-checkpoint inhibitors, which target immune checkpoints such as cytotoxic lymphocyte-associated protein 4 (CTLA-4) or programmed cell death protein 1 (PD-1) and its ligand programmed death-ligand 1 (PD-L1), as well as chimeric antigen receptor T cell (CAR-T) therapy.