CTLA4 and head and neck squamous cell carcinoma: CTLA-4 (cytotoxic T-lymphocyte associated protein 4, also known as cluster of differentiation 152, CD152) and programmed cell death protein 1 (PD-1) (and its ligands PD-L1 and PD-L2) are immune checkpoints targeted by humanized antibodies for the treatment of HNSCC.