PDCD1 and head and neck cancer: Particularly, the blockade of immune checkpoints CTLA4 (cytotoxic T-lymphocyte-associated protein 4), PD1 (programmed cell death protein 1), and PD-L1 (programmed cell death 1 ligand) has clinical successes in various cancers including melanoma, lung cancer, renal cell cancer, bladder cancer, head and neck cancer, Hodgkin's lymphoma, and the cancers with MSI (microsatellite instability) or DNA mismatch-repair deficiency [6].