Immune checkpoint inhibitors, such as cytotoxic T lymphocyte-associated antigen-4 inhibitors, programmed cell death protein-1 inhibitors, and programmed cell death ligand-1 inhibitors, etc., can be used for the treatment of various malignant tumors, such as lung cancer, colorectal cancer, gastric cancer, melanoma, renal cell carcinoma, Hodgkin lymphoma, etc (19–22). The gene discussed is PDCD1; the disease is gastric cancer.