APTX and Nijmegen breakage syndrome: Indeed, mutation of the genes encoding DNA ligase IV (LIG4), Nibrin (NBS1) or Aprataxin (APTX) results in DDR disorders, specifically LIG4 syndrome, Nijmegen breakage syndrome (NBS) and Ataxia oculomotor apraxia-1, in association with increased radiosensitivity [25–27].