CD274 and cancer: The top three most widely used checkpoint inhibitor classes target immune inhibitory receptors: cytotoxic T lymphocyte-associated molecule-4 (CTLA-4), programmed cell death receptor-1 (PD-1), and programmed cell death receptor ligand-1 (PD-L1).4Figure 1 demonstrates the ligand-receptor interactions involved in immunological recognition of cancer cells on a cellular level.