PDCD1 and head and neck squamous cell carcinoma: Based on these above results, we studied the relationship between PER2, PER3 and immune infiltrating cells, which showed that the high expression of PER3 mRNA was positively correlated with the immune score (r = 0.16, p < 0.001), T cells regulatory (Tregs) levels (r = 0.10, p = 0.028), PDCD1 (r = 0.22, p < 0.001), T cells CD4 + memory activated (r = 0.10, p = 0.023), T cells CD8 + (r = 0.09, p = 0.036), T cells follicular helper (r = 0.12, p = 0.009) in HNSCC (Supplementary Fig. 2A).