Significant progress has been made in recent years in elucidating the molecular controls of cellular responses to DNA damage in mammalian cells. Much of our understanding of the mechanisms involved in cellular DNA damage response pathways has come from studies of human cancer susceptibility syndromes that are altered in DNA damage responses. Ataxia-telangiectasia mutated (ATM), the gene mutated in the disorder ataxia-telangiectasia, codes for a protein kinase that is a central mediator of responses to DNA double-strand breaks (DSB) in cells. Once activated, ATM phosphorylates numerous substrates in the cell that modulate the response of the cell to the DNA damage. We recently developed a novel system to create DNA DSBs at defined endogenous sites in the human genome and used this system to detect protein recruitment and loss at and around these breaks by chromatin immunoprecipitation. Results from this system showed the functional importance of ATM kinase activity and phosphorylation in the response to DSBs and supported a model in which ordered chromatin structure changes that occur after DNA breakage and that depend on functional NBS1 and ATM facilitate DNA DSB repair. Insights about these pathways provide us with opportunities to develop new approaches to benefit patients. Examples and opportunities for developing inhibitors that act as sensitizers to chemotherapy or radiation therapy or activators that could improve responses to cellular stresses, such as oxidative damage, are discussed. Relevant to the latter, we have shown benefits of an ATM activator in disease settings ranging from metabolic syndrome to cancer prevention.