PDCD1 and neoplasm: In detail, cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4)/B7-1 or B7-2 and programed cell death 1 (PD-1)/its ligands programed cell death 1 ligand 1 (PD-L1) or PD-L2 axis are recognized to be the most important central checkpoints in the immune invasion for tumor progression [4].