PDCD1 and neoplasm: Targeting of immune checkpoint inhibitor (ICI) proteins including cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and program cell death 1 (PD-1) or its ligand, PD-L1, is able to partially reverse cancer immunosuppression of tumor-specific cytotoxic effector cells by unleashing them to kill tumor cells.