PDCD1 and neoplasm: These monoclonal antibodies block proteins that down-regulate immune responses (e.g., cytotoxic T-lymphocyte antigen 4 [CTLA-4] and programmed cell death 1 [PD-1]) or their ligands (e.g., programmed cell death ligand 1 [PD-L1]), resulting in activation of the immune system and enhancing recognition of tumor cells [6].