PDCD1 and neoplasm: The binding of programmed cell death protein 1 receptor PD-1 (CD279), expressed by cytotoxic T lymphocytes (CTLs) and other immune effectors, to its ligand PD-L1 (B7-H1, CD274), expressed by myeloid and cancer cells in the tumor microenvironment (TME), limits CTL expansion, activation, cytotoxicity, and survival, driving immune evasion [1,2].