CTLA4 and head and neck squamous cell carcinoma: The fact that HNSCC and other advanced HNC present an extremely immunosuppressive tumour microenvironment, high tumour mutation burden and high expression of immune checkpoint inhibitors (programmed cell death protein 1, PD-1; and cytotoxic T-lymphocyte-associated protein 4, CTLA4) makes them very attractive candidates for cancer immunotherapy [96,97].