PDCD1 and cancer: Among the available immunotherapy options, therapeutic antibodies targeting immune checkpoint blockage, including programmed cell death 1 (PD-1) and its ligand PD-ligand 1 (PD-L1) and cytotoxic T lymphocyte-associated protein 4 (CTLA-4), have been shown to be a promising approach as anti-cancer treatment by reinvigorating immune responses against cancers [15,16,17].