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Genetic and cytologic data from Saccharomyces cerevisiae and mammals implicate the Mre11 complex, consisting of Mre11, Rad50, and Nbs1, as a sensor of DNA damage, and indicate that the complex influences the activity of ataxia-telangiectasia mutated (ATM) in the DNA damage response. Rad50S/S mice exhibit precipitous apoptotic attrition of hematopoietic cells. We generated ATM- and Chk2-deficient Rad50S/S mice and found that Rad50S/S cellular attrition was strongly ATM and Chk2 dependent. The hypomorphic Mre11ATLD1 and Nbs1ΔB alleles conferred similar rescue of Rad50S/S-dependent hematopoietic failure. These data indicate that the Mre11 complex activates an ATM-Chk2-dependent apoptotic pathway. We find that apoptosis and cell cycle checkpoint activation are parallel outcomes of the Mre11 complex-ATM pathway. Conversely, the Rad50S mutation mitigated several phenotypic features of ATM deficiency. We propose that the Rad50S allele is hypermorphic for DNA damage signaling, and that the resulting constitutive low-level activation of the DNA damage response accounts for the partial suppression of ATM deficiency in Rad50S/S Atm-/- mice. [ABSTRACT FROM AUTHOR]