PDCD1 and head and neck squamous cell carcinoma: First, a correlation analysis was performed between the MRGPI and immune checkpoint proteins, including PD-1 (PDCD1), PD-L1 (CD274), CTLA-4, lymphocyte activation gene-3 (LAG-3), T-cell immunoglobulin, and immunoreceptor tyrosine-based inhibition motif domain (TIGIT) in the 502 HNSCC patients.