CTLA4 and head and neck cancer: The dual checkpoint blockade of PD-1/PD-L1 and CTLA-4 in advanced head and neck cancer patients and low or no PD-L1 tumour cell expression is currently being investigated in different studies, e.g., CheckMate-651 (NCT02741570), CONDOR (NCT02319044), EAGLE (NCT02369874) and KESTREL (NCT02551159) [452,453,454,455,456,457,458].