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). This evidence concerns the gene PDCD1 and cancer.