Ataxia-telangiectasia is a rare clinical disorder manifesting a variety of different abnormalities, including progressive neurodegeneration, increased cancer incidence, immune deficiency, sterility, and extreme radiosensitivity. Recent studies have demonstrated that the defective gene product in this disease, ATM, is a protein kinase. The identification of several different substrates for this kinase is beginning to explain the wide array of different physiologic abnormalities that occur when this gene product is dysfunctional. Since the ATM protein is a critical signaling molecule in the cellular response to ionizing irradiation, the identification of these substrates also results in elucidation of the steps involved in a number of different cellular signaling pathways initiated by irradiation. Such insights also result in the identification of potential new targets for enhancing the efficacy of radiation therapy.