PRKDC and head and neck squamous cell carcinoma: One study demonstrated that siRNA knockdown of DNA-Pk significantly decreased the survival of two HPV-negative HNSCC cell lines (UTSCC15 and UTSCC45) post-irradiation, because of the persistence of DSBs as revealed by γH2AX/53BP1 foci increases at 24 h post-irradiation (Ref.