PDCD1 and cancer: Immune checkpoint inhibitors (ICIs) have recently been a source of promising new cancer treatments that increase the cytotoxicity of T cells by blocking intrinsic downregulators of immunity such as cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed cell death 1 (PD-1) or its ligand of programmed cell death ligand 1 (PD-L1) so as to enhance antitumor activity (9).