PDCD1 and cancer: An increasing number of immune checkpoint inhibitors, including the monoclonal antibodies anti-cytotoxic T lymphocyte antigen 4 (anti-CTLA-4), anti-programmed cell death protein 1 (anti-PD-1), and anti-programmed cell death ligand-1 (anti-PD-L1), have been shown to be effective in the treatment of cancers (13).