PDCD1 and cancer: Immune checkpoint inhibitors (ICIs), including anti-programmed cell death protein-1 (PD-1), anti-programmed cell death ligand-1 (PD-L1), anti-cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), and lymphocyte activation gene-3 (LAG-3) antibodies, have become an integral component of treatment for more than 20 types of cancer by releasing inhibitory signals on antitumor immunity and enhancing T cell–mediated immune responses (1).