The ATM protein kinase is a primary activator of the cellular response to DNA double-strand breaks (DSBs). In response to DSBs, ATM is activated and phosphorylates key players in various branches of the DNA damage response network. ATM deficiency causes the genetic disorder ataxia-telangiectasia (A-T), characterized by cerebellar degeneration, immunodeficiency, radiation sensitivity, chromosomal instability and cancer predisposition. The MRN complex, whose core contains the Mre11, Rad50 and Nbs1 proteins, is involved in the initial processing of DSBs. Hypomorphic mutations in the NBS1 and MRE11 genes lead to two other genomic instability disorders: the Nijmegen breakage syndrome (NBS) and A-T like disease (A-TLD), respectively. The order in which ATM and MRN act in the early phase of the DSB response is unclear. Here we show that functional MRN is required for ATM activation, and consequently for timely activation of ATM-mediated pathways. Collectively, these and previous results assign to components of the MRN complex roles upstream and downstream of ATM in the DNA damage response pathway and explain the clinical resemblance between A-T and A-TLD.