PDCD1 and gastric cancer: Immune checkpoint inhibitors (ICIs) enhance antitumor activity by blocking immune-intrinsic downregulating factors such as cytotoxic T-lymphocyte antigen 4 (CTLA-4), programmed cell death 1 (PD-1), and programmed cell death ligand 1 (PD-L1), which made breakthroughs in the treatment of a series of tumors such as gastric cancer, liver cancer, and non-small cell lung cancer (2–4).