PDCD1 and gastric cancer: Immunotherapy with immune checkpoint inhibitors (ICIs), such as monoclonal antibodies (mAbs), targeting the programmed cell death protein 1 (PD-1), its ligand (PD-L1), and cytotoxic T-lymphocyte antigen 4 (CTLA-4) is the new standard treatment for cancers, such as lung cancer, malignant melanoma, gastric cancer, lymphoma, urothelial carcinoma, and renal cell carcinoma (RCC) [1–3].