HNSCC tumor tissue samples contained a significantly increased fraction of PD-1 expressing T cells (55.7 ± 19.0%) compared to PBMC HNSCC (18.0 ± 11.2%; p < 0.0001), but not compared to non-cancerous mucosa (32.6 ± 22.2%). Here, PDCD1 is linked to head and neck squamous cell carcinoma.