Clearly the MRN complex plays a very important role in ATM function, particularly considering the similarity in clinical phenotypes of patients lacking ATM (A-T) compared with patients expressing hypomorphic alleles of MRE11 or RAD50 (A-T-like Disorder) or Nbs1 (Nijmegen Breakage Syndrome) (5). Here, ATM is linked to Nijmegen breakage syndrome.