Since ICIs act by releasing the breaks on pre-existing anti-tumor T cell immunity, the lower response rate of HNSCC patients to anti-programmed cell death protein 1 (PD-1 aka CD279) antibody suggests that a large number of HNSCC patients either have low pre-existing anti-tumor immunity or the tumors use immunosuppressive pathways that can inhibit T cell activity. This evidence concerns the gene PDCD1 and head and neck squamous cell carcinoma.