Ataxia telangiectasia (AT) is a developmental disorder in which many organ systems are affected. The children are recognized by a progressive cerebellar deterioration. The gene for AT has now been localized to a region of chromosome 11q22-23 of no more than 3Mb in size and its product appears to be involved directly or indirectly in some form of DNA recombination. Patients and their cells are unusually sensitive to ionizing radiation and various radiometric drugs. Observations on the progressive nature of the disorder, with loss of selected cells or failure to develop normally, might be compatible with the pathological effect of an inability to correctly regulate apoptosis in some cell lineages. While this is an intriguing speculation, there is, at present, no evidence for such a defect in AT. (author).