Indeed, evidence has demonstrated that inhibitors against enzymes such as poly(ADP-ribose) polymerase 1 (PARP-1), ataxia–telangiectasia mutated (ATM), ataxia telangiectasia and Rad3-related (ATR), and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) can enhance the radiosensitivity of HNSCC cells, at least in vitro [12,13,14,15,16]. Here, ATR is linked to head and neck squamous cell carcinoma.