PDCD1 and neoplasm: The first evidence of PD-1 signaling pathway implicated in mediating tumor immunity was noted in 2002 [25], in which the overexpression of the ligand of PD-1, PD-L1 [26,27], was found to impair the cytolytic activity of T cells and markedly enhance tumorigensis and tumor invasiveness, and additionally such effects could be reversed by anti-PD-L1 treatment through applying anti-PD-L1 monoclonal antibodies.