PDCD1 and gastric cancer: Currently, three main types of checkpoint inhibitor immunotherapy drugs, i.e. anti-CTLA-4, PD-1, and PD-L1 inhibitors, have been developed and demonstrated to be effective treatment for a variety of malignant tumor types, where the anti-PD-L1 inhibitor has been approved for advanced GC patients tested with positive PD-L1 expression [85, 86].