To review what the cloning of the ATM gene has told us about ataxia telangiectasia (A-T). Perhaps the most interesting and potentially useful finding following the cloning of the gene has been the discovery of homologies with various previously known PIK related kinases. There is also a very wide range of mutations associated with A-T and it is clear that some ATM mutations can lessen the severity of both the clinical and cellular features of the disorder. The function(s) of ATM remain an enigma. There are several indications of its involvement in protecting the cell from the effects of ionizing radiation and radiomimetic agents, although the pathways that involve p53 and c-Abl are far from clear. Expression of AIM in tissue culture cells can reverse the cellular features of A-T, particularly the increased level of radiosensitivity. The localization of ATM on meiotic chromosomes and its speculated role in meiotic recombination is an important finding. Finally, cloning the ATM gene has allowed the development of mouse models, which are providing information about A-T and will be crucial for testing future treatments for the disorder.