Interestingly though, when cells were exposed to a prolonged period of 4-5 days in hypoxia (1% oxygen) prior to irradiation, ATM-deficient HNSCC cells showed increased radiosensitivity associated with a delayed phosphorylation of DNA-Pkcs and delayed NHEJ pathway activation, whilst the other two HPV-negative HNSCC cell lines displayed increased radioresistance[49]. This evidence concerns the gene ATM and head and neck squamous cell carcinoma.