Much efforts has been invested to exploit the efficacy of treating cancer patients with fully-humanized monoclonal antibodies against two of the most widely studied immune checkpoint regulators—cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and programmed cell death protein-1 (PD-1) or its ligand PD-1-ligand 1 (PD-L1). Here, PDCD1 is linked to cancer.