PDCD1 and neoplasm: Currently approved immune checkpoint inhibitors (ICI) are monoclonal antibodies that act by reversing tumor escape caused by two negative regulators of tumor immunity: cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed cell death 1 (PD-1) or its ligand, programmed cell death ligand 1 (PD-L1).