PDCD1 and cancer: ICIs, including cytotoxic T lymphocyte protein 4 (CTLA-4), programmed cell death 1 (PD-1), and programmed cell death ligand 1 (PD-L1), mainly target proteins that negatively regulate T cell-meditated host immune response to cancer, thus enabling immune activation and antitumor response (Haslam and Prasad, 2019).