ATM and head and neck squamous cell carcinoma: Our observations in the ATM-deficient HNSCC cell line SKX, which showed higher radiosensitivity to protons relative to photons, prompted us to evaluate the efficacy of pharmacological inhibitors targeting a number of key enzymes of the two major DSB repair pathways non-homologous end joining (NHEJ) and homologous recombination (HR) [23].