GPR18 and head and neck squamous cell carcinoma: After analyzing the discovery and validation cohorts, we identified that eight genes (S100A10, FABP4, HMGA2, SLC7A2, GPR18, PPP1R1C, GDNF, and MAP2) were associated with OS in HPV-positive HNSCC patients in both cohorts (p value < 0.05) (Supplementary Table S5).