PDCD1 and head and neck squamous cell carcinoma: PD-1 inhibitors such as nivolumab and pembrolizumab have been approved to treat metastatic melanoma, metastatic non-small cell lung cancer, renal cell carcinoma, classical Hodgkin’s lymphoma, head and neck squamous cell carcinoma (SCC), urothelial carcinoma, hepatoma previously treated with sorafenib, and metastatic gastric or gastroesophageal junction adenocarcinoma [9].