PDCD1 and head and neck squamous cell carcinoma: Response rates following ICI therapy (anti-CTLA4 or anti-PD-1/PD-L1 antibody therapy) were observed in 13.3 to 44% of patients with head and neck squamous cell carcinoma (HNSCC) [5], advanced melanoma [6–11], non-small cell lung cancer [12–14], renal cell carcinoma [15, 16], Merkel cell carcinoma [17] and metastatic urothelial carcinoma [18].