It has been proven that monoclonal antibodies targeting these immune checkpoints, such as ipilimumab for CTLA-4 and nivolumab and pembrolizumab for PD-1, exert clear-cut beneficial effects on the clinical course of cancer, including metastatic melanoma, non-small cell lung cancer, renal cell carcinoma, and Hodgkin lymphoma [8,9,10]. Here, PDCD1 is linked to cancer.