PDCD1 and head and neck squamous cell carcinoma: Moreover, we found that the expression of the main immune checkpoint genes, such as PDCD1, CTLA4, TIGIT, and HAVCR2, positively correlated with ZC3H12D expression and was upregulated in HNSCC patients with high ZC3H12D expression, indicating that patients with high ZC3H12D expression may respond to immunotherapy better.