Originally identified in the same pathway as ATM based on its response to ionizing radiation (IR)-induced DSBs, the MRE11/RAD50/NBS1 (MRN) complex has been associated with radiation hypersensitivity syndromes, A-T-like disease (ATLD, MRE11 mutation) [101] and Nijmegen breakage syndrome (NBS or Nibrin, NBS1 mutation) [102,103,104]. This evidence concerns the gene ATM and Nijmegen breakage syndrome.