PDCD1 and gastric cancer: The use of the immune checkpoint programmed cell death-1 (PD-1) blocking agents, the blockage of its ligands (PD-L1 or B7-H1), as well as the cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), have shown remarkable therapeutic efficacy against several solid tumors [39], and may also represent a significant advance in GC treatment [40].