PDCD1 and cancer: The US Food and Drug Administration (FDA) has approved four immune checkpoints, including cytotoxic T-lymphocyte-associated protein-4 (Cytotoxic T-lymphocyte associated protein-4,CTLA-4), Programmed cell death-1 (PD-1), Programmed death ligand-1 (PD-L1), and Lymphocyte activation gene-3 (LAG-3) which are immune checkpoint inhibitors that have shown promising results in cancer treatment (39).