Cells from patients with ataxia telangiectasia (AT) are hypersensitive to ionizing radiation and are defective in the regulation of DNA synthesis. A complementary DNA that corrects the radiation sensitivity and DNA synthesis defects in fibroblasts from an AT group D patient was isolated by expression cloning and shown to encode a truncated form of I kappa B-alpha, an inhibitor of the nuclear factor kappa B (NF-kappa B) transcriptional activator. The parental AT fibroblasts expressed large amounts of the I kappa B-alpha transcript and showed constitutive activation of NF-kappa B. The AT fibroblasts transfected with the truncated I kappa B-alpha expressed normal amounts of the I kappa B-alpha transcript and showed regulated activation of NF-kappa B. These results suggest that aberrant regulation of NF-kappa B and I kappa B-alpha contribute to the cellular defect in AT.