Immune checkpoint inhibitors (ICI), including antibodies against cytotoxic T-lymphocyte associated antigen-4 (CTLA-4), programmed cell death-1 (PD-1), and programmed-death-ligand-1 (PD-L1), function to help the body overcome potential barriers to the recognition and elimination of cancer by the host immune system. Here, PDCD1 is linked to cancer.