A variety of genetic disorders involve genome instability and abnormal response to DNA damaging agents. Investigation of these disorders has revealed different metabolic pathways responsible for damage repair on one hand, and for signaling the presence of the damage to cellular regulatory systems on the other hand. Ataxia-telangiectasia (A-T) is a typical example of such a disorder. This autosomal recessive disease is characterized by degeneration of the cerebellum, thymus, and gonads, immunodeficiency, premature aging, cancer predisposition, and acute sensitivity to ionizing radiation. A-T cells show hypersensitivity to ionizing radiation and radiomimetic chemicals as well as defects in various signal-transduction pathways induced by these agents, most notably the activation of cell cycle checkpoints (1–3).