PDCD1 and head and neck squamous cell carcinoma: A six-gene IFN-γ signature that was reported to induce programmed death ligand 1 (PD-L1) expression and predict the therapeutic efficacy of the PD-1 inhibitor pembrolizumab in head and neck squamous cell carcinoma (34) was also significantly enriched in Immune Class (P = 1.6e-04).