The use of checkpoint inhibitors, such as therapeutic monoclonal antibodies targeting cytotoxic T-Lymphocyte Associated Protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), and programmed death ligand-1 (PD-L1) has greatly improved the survival of many cancer patients, especially with metastatic melanoma (82). Here, PDCD1 is linked to cancer.