CD274 and head and neck squamous cell carcinoma: Other than programmed death ligand-1 (PD-L1) expression — which is used in HNSCC to stratify immunotherapy with pembrolizumab monotherapy or in combination with chemotherapy — biomarkers such as CD8+ T cell infiltration, tumor mutational burden, or immune-gene expression profiling have been explored, but have yet to be validated in this disease (7).