PD-1 expression in the CD4+ and CD8+ T-cell subsets in HNSCC tumors was not significantly different comparing early and late stage (52.3 ± 20.3% vs. 55.1 ± 19.6% and 50.6 ± 25.0% vs. 65.3 ± 22.7%; Figure 5D, left plot). Here, PDCD1 is linked to head and neck squamous cell carcinoma.