CD274 and head and neck squamous cell carcinoma: The degree of PD-L1 expression in HNSCC tissues is varied from study to study, which may be for several reasons, such as different protocols for immunohistochemical staining, different antibodies with varying binding affinities, the inconsistent cut-off for positivity, biopsy quality, subjective factors of pathologists' evaluation and intratumor heterogeneity (45).