The transcription factor NF-kappaB modulates apoptotic responses induced by genotoxic stress. We show that NF-kappaB essential modulator (NEMO), the regulatory subunit of IkappaB kinase (IKK) (which phosphorylates the NF-kappaB inhibitor IkappaB), associates with activated ataxia telangiectasia mutated (ATM) after the induction of DNA double-strand breaks. ATM phosphorylates serine-85 of NEMO to promote its ubiquitin-dependent nuclear export. ATM is also exported in a NEMO-dependent manner to the cytoplasm, where it associates with and causes the activation of IKK in a manner dependent on another IKK regulator, a protein rich in glutamate, leucine, lysine, and serine (ELKS). Thus, regulated nuclear shuttling of NEMO links two signaling kinases, ATM and IKK, to activate NF-kappaB by genotoxic signals.