PDCD1 and head and neck squamous cell carcinoma: Significant expression differences were observed in PDCD1 (P < 0.05), PDCD1LG2 (P < 0.05), TIGIT (P < 0.05), and ITPRIPL1 (P < 0.001) between the high- and low-risk groups in HNSCC (Figure 9A).