ATR and cancer: Radiation therapy combined with targeted DSB damage repair protein inhibitors has the potential for clinical application in patients with cancer; current drug development and clinical trials for targets such as DNA-PKcs, ATM/ATR (i.e., ataxia-telangiectasia mutated and Rad3-related protein kinases), and the MRN (i.e., MRE11-Rad50-NBS1) complex are underway and may, we hope, better explain the poor efficacy caused by radiotherapy resistance (27).