CD274 and head and neck squamous cell carcinoma: Several immunosuppressive checkpoints have been reported to be associated with HNSCC, including programmed death-1 (PD-1), programmed death ligand-1 (PD-L1), cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), lymphocyte activation gene-3 (LAG-3), T cell immunoglobulin mucin-3 (TIM-3), and T cell immunoglobulin ITIM domain (TIGIT) (Moy et al., 2017; Deng et al., 2018).