PDCD1 and head and neck cancer: Clinical immunotherapy with monoclonal antibodies to block the CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) or programmed cell death protein 1 (PD-1)–PD-1 ligand (PD-L1) axes have been FDA-approved for the treatment of several malignancies such as melanoma, non-small cell lung cancer, RCC, Hodgkin lymphoma, Merkel cell carcinoma, head and neck cancer and carcinoma of the bladder.