Patients with ataxia-telangiectasia (AT), a human autosomal recessive genetic disease, are uniformly hypersensitive to ionizing radiation as measured by colony-forming ability and by chromosomal aberrations. Obligate heterozygotes, i.e., parents of AT patients, are slightly more radiosensitive than normal humans in terms of both colony-forming ability and chromosomal aberrations formed in G2. Thus, this system not only furnishes a model system to study factors that are responsible for radioresistance in normal human beings, but is also a unique tool for determining the role of gene dosage on radiation-induced cell killing. Because AT cells seem to be hypomutable to ionizing radiation, they also can be used to study the relationship between radiosensitivity and mutability and, therefore, carcinogenesis. Isolation of the defective gene that causes hypersensitivity in AT cells and its counterpart in normal cells should lead to a breakthrough in our understanding of radiation effects and how they can be prevented in human beings.