PDCD1 and head and neck cancer: In head and neck cancer, both clinical data and murine models demonstrate that the expression of PD-1 on TILs correlates with clinical outcomes and response to immunotherapy, with tumor rejection in response to anti-PD-1 monoclonal antibody (mAb) therapy associated with a decrease in PD-1high and an increase in PD-1low CD8+ TILs [101].