PDCD1 and cancer: Immunotherapy or immune checkpoint inhibitors (ICI) are monoclonal antibodies that block the immune inhibiting checkpoints, such as programmed cell death protein 1 (PD-1), programmed death ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), and restore the function of the immune system to target cancer cells.