HLA-E and head and neck squamous cell carcinoma: Parallel biological assays demonstrated that in the paired treatment-naïve HNSCC tumor and plasma samples (N = 20), PEX HLA-E mRNA (a non-survivor-predicting marker) was positively corelated with overexpression of HLA-E protein (P = 0.0191) and the dense population of tumor-infiltrating NK cells (P = 0.024) in the corresponding tumor, suggesting that the HLA-E-NKG2A immune checkpoint may inhibit the antitumor effect of PD-1blockade.