A wide spectrum of investigators are interested in A-T 2 because of the pleiotropic effects of the gene. Patients show a progressive cerebellar degeneration, first evident in infancy, resulting in severe ataxia, dysarthria, and oculomotor dyspraxia. All patients show a defect of cell mediated immunity and many show a significant deficiency in humoral immunity. There is a greatly increased predisposition to both lymphoma and lymphoid leukemia. T-cell leukemia appears to be much more common in A-T patients than in the general population. In addition there is an excess of particular epithelial cell tumors including carcinoma of the stomach, liver, and ovaries. A-T patients are unusually sensitive to the effects of ionizing radiation. This increased radiosensitivity can be observed at the cellular level as a reduced colony forming ability of cultured fibroblasts, compared with normal, following exposure to Xor ),-rays. Another intriguing response to radiation can be measured as the effect on DNA synthesis. Following exposure of normal cells to ),-rays DNA synthesis is inhibited in a biphasic manner with an initial steep component. In irradiated A-T cells there is not the same reduction in the rate of DNA synthesis and levels remain high. This is termed radioresistant DNA synthesis. The responses of A-T cells to ionizing radiation have provided a means for assaying the cellular phenotype and in analyzing genetic heterogeneity (see below). The Fifth International Workshop was organized by Dr. R. A. Gatti (University of California, Los Angeles, Los Angeles, CA). The major aim of the Workshop was to discuss progress toward mapping and cloning A-T gene(s). Other important sessions included discussion on the possibility that the A-T mutation produces a defect in recombination, the role of the A-T gene in the heterozygous state in cancer predisposition, the status of heterozygote detection, and the range of clinical heterogeneity in patients.