CD274 and head and neck squamous cell carcinoma: Since it was known that radiotherapy could induce the PD-L1 in several cancer cells including head and neck squamous cell carcinoma, bladder cancer and non-small cell lung cancer [9–11], it was supposed that PD-1/PD-L1 axis blockade was required to inhibit the adverse effect of radiotherapy and may be benefit to treat cancer patients.