PDCD1 and cancer: In particular, checkpoint blockade, such as the use of monoclonal antibodies (mAbs) against cytotoxic T-lymphocyte associated protein 4 (CTLA-4), programmed death ligand 1 (PD-L1), and programmed cell death 1 (PD-1), has shown impressive potential to treat different cancer types by relieving the suppression from the inhibitory checkpoints in the tumor microenvironment (TME) (1, 2).