ATR and head and neck squamous cell carcinoma: Therefore, using specific inhibitors against the protein kinases ataxia telangiectasia mutated (ATM), ataxia telangiectasia and Rad3-related (ATR) and the DNA-dependent protein kinase catalytic subunit (DNA-Pkcs), we investigated their impact in radiosensitising HPV-negative HNSCC cells to PBT of increasing LET.