CD274 and head and neck squamous cell carcinoma: This reflects the problems derivative from the heterogeneity of tumours included in different studies (HNSCC, OPSCC, HPV status), the associated patient treatment schedules, as well as the reagents and detection methodologies, the definitions of positivity, the cut-off levels used, and the scoring system with inclusion of tumour cells alone (TPS: tumour proportion score) or all relevant cells expressing PD-L1 (CPS: combined proportion score).