PDCD1 and neoplasm: Co-immunosuppressive/co-inhibitory molecules in the TME, including programmed cell death protein 1 (PD-1), cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) (CD152), lymphocyte-activation gene 3 (LAG-3), 2B4, CD160, and TIM-3, can dampen the antitumor mechanism by helping tumor cells escape from host immune surveillance.