To date, ICIs targeting three different immune checkpoints have been approved by the US Food and Drug Administration (FDA) for cancer therapy, namely cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) antibody, programmed cell death protein 1 (PD-1) antibody, and programmed death ligand 1 (PD-L1) antibody. Here, PDCD1 is linked to cancer.