PDCD1 and gastric cancer: Recent discovery and development of the immunotherapy agents, specifically the checkpoint inhibitors, such as inhibitors against programmed cell death protein 1 (PD-1), programmed death-ligand 1 (PD-L1) or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) that can restore T-cell function to destroy tumor cells, have been proposed as promising options for the targeted treatment of GC or EC [12–14].