CTLA4 and cancer: Checkpoint inhibitors are monoclonal antibodies targeting regulation receptors in the immune system such as programmed cell death receptor 1 (PD-1), programmed cell death ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4); binding to the inhibitory receptor mediates the immune system to attack and destroy cancer cells.