PDCD1 and cancer: Immune checkpoint inhibitors (ICIs), including anti-programmed-death-1 (PD-1), anti-programmed-death-ligand-1 (PD-L1, also called CD274), and anti-cytotoxic-T-lymphocyte-associated-protein-4 (CTLA-4) monoclonal antibodies, have shown durable antitumor activity in various cancer types by reactivating the ability of immune cells to recognize and eliminate tumor cells [8].