CD274 and head and neck squamous cell carcinoma: Additionally, testing of PD-1/PD-L1 inhibitors in early-phase trials has been accompanied by the parallel development of companion diagnostic assays with which to evaluate PD-L1 immunohistochemical staining of immune cells and/or certain tumor cells, such as NSCLC and squamous cell carcinoma of the head and neck (SCCHN) [34].