Cells depend on a checkpoint mechanism that stops cell division when damaged DNA is detected, but which molecular sensors "see" breaks in DNA strands is unclear. DNA damage leads to activation of the protein kinase ATM, the product of the gene mutated in ataxia-telangiectasia (a human disease characterized by sensitivity to radiation and predisposition to cancer). Lee and Paull (see the Perspective by Abraham and Tibbetts) describe an in vitro assay in which dimeric ATM (but not the monomeric enzyme) is activated by broken DNA molecules in the presence of the MRN complex, a complex of three other proteins. The MRN complex interacts with DNA and recruits ATM to the DNA-bound complex. Thus, adenosine triphosphate-dependent DNA unwinding by the MRN complex appears to be a necessary step in the activation of ATM.