PDCD1 and cancer: ICIs targeting cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed cell death protein-1 (PD-1) and its ligand programmed death ligand-1 (PD-L1) [7] have demonstrated a significant and consistent benefit to survival when compared with standard treatments in prospective randomized clinical trials [8,9,10,11,12], leading to regulatory approval for several cancer types, including HNSCC.