The ATM protein kinase is a key regulator of the cellular response to DNA double-strand breaks (DSBs). It is essential for activating DNA damage response pathways and is deficient in ataxia-telangiectasia (A-T), a genetic disorder characterized by cerebellar degeneration, immunodeficiency, and cancer predisposition. The MRN complex, consisting of Mre11, Rad50, and Nbs1, is involved in the initial processing of DSBs. Mutations in NBS1 and MRE11 lead to Nijmegen breakage syndrome (NBS) and A-T-like disease (A-TLD), respectively. The study shows that functional MRN is required for ATM activation, which is essential for timely activation of ATM-mediated pathways. The results suggest that components of the MRN complex play roles both upstream and downstream of ATM in the DNA damage response pathway, explaining the clinical similarities between A-T and A-TLD. The study demonstrates that MRN deficiency leads to impaired ATM activation and phosphorylation of downstream targets. Reconstitution of the MRN complex in A-TLD(S) cells restores ATM activation, indicating that functional MRN is necessary for proper ATM activation. The nuclease activity of Mre11 is specifically required for ATM activation, as a nuclease-defective Mre11 mutant failed to fully restore ATM activation. These findings highlight the importance of the MRN complex in the early stages of the DNA damage response and its role in ATM activation. The study also shows that the MRN complex is essential for the initial processing of DSBs and for the recruitment of damage response proteins to the sites of DNA damage. The results suggest that the clinical resemblance between A-T and A-TLD may reflect a dependence of ATM activation on functional MRN. The study provides insights into the complex interplay between the MRN complex and ATM in the DNA damage response pathway.The ATM protein kinase is a key regulator of the cellular response to DNA double-strand breaks (DSBs). It is essential for activating DNA damage response pathways and is deficient in ataxia-telangiectasia (A-T), a genetic disorder characterized by cerebellar degeneration, immunodeficiency, and cancer predisposition. The MRN complex, consisting of Mre11, Rad50, and Nbs1, is involved in the initial processing of DSBs. Mutations in NBS1 and MRE11 lead to Nijmegen breakage syndrome (NBS) and A-T-like disease (A-TLD), respectively. The study shows that functional MRN is required for ATM activation, which is essential for timely activation of ATM-mediated pathways. The results suggest that components of the MRN complex play roles both upstream and downstream of ATM in the DNA damage response pathway, explaining the clinical similarities between A-T and A-TLD. The study demonstrates that MRN deficiency leads to impaired ATM activation and phosphorylation of downstream targets. Reconstitution of the MRN complex in A-TLD(S) cells restores ATM activation, indicating that functional MRN is necessary for proper ATM activation. The nuclease activity of Mre11 is specifically required for ATM activation, as a nuclease-defective Mre11 mutant failed to fully restore ATM activation. These findings highlight the importance of the MRN complex in the early stages of the DNA damage response and its role in ATM activation. The study also shows that the MRN complex is essential for the initial processing of DSBs and for the recruitment of damage response proteins to the sites of DNA damage. The results suggest that the clinical resemblance between A-T and A-TLD may reflect a dependence of ATM activation on functional MRN. The study provides insights into the complex interplay between the MRN complex and ATM in the DNA damage response pathway.