PDCD1 and cancer: Programmed cell death 1 (PD-1, also known as CD279) T cell receptor and its ligands—programmed death ligand 1 (PD-L1 or B7-H1 or CD274) and programmed death ligand 2 (PD-L2 or B7-DC or CD273)—are the prime immune checkpoint pathway components, which are targeted for immune checkpoint inhibitory therapy against cancer [13,15].