Ataxia telangiectasia (AT) is a rare, autosomal recessive disorder that manifests as a pleiotropic developmental defect with symptoms including a progressive cerebellar ataxia, thymic hypoplasia, variable immunodeficiencies, hypogonadism, and telangiectasias of blood vessels in the eyes, hands and face. Patients with AT show marked radiosensitivity to ionizing radiation, and AT-derived cell lines are defective in G1S, S and G2M cell cycle checkpoints. AT patients have a greatly enhanced predisposition for tumor development, particularly lymphoid malignancies (Sedgwick and Boder 1991; Taylor et al. 1998).