PDCD1 and head and neck squamous cell carcinoma: Use of immune checkpoint inhibitors such as anti-programmed cell death protein 1 (PD-1) or anti-programmed death-ligand 1 (PD-L1), is already approved for clinical use for metastatic/recurrent HNSCC, but only one in five patients responds to this type of therapy (Vermorken et al., 2008), and much effort is currently placed to understand this and to identify biomarkers of response of immunotherapy in HNSCC.