The cloning of ATM in 1995, the gene responsible for ataxia-telangiectasia, opened a dimension of biological research that is as complex and intriguing to cell biologists as this classic disorder has been to clinicians for four decades. The phenotype is both variable and stereotyped, with significant differences between patients in the rate of progression or appearance of the multiple features yet consistent in their characteristic nature. Ataxia telangiectasia usually has been misdiagnosed for the first few years of life, while accurate diagnosis might most impact family planning. Newly produced ATM-deficient transgenic mice express most of the cellular features of the disorder but have yet to mimic the distinctive neurodegeneration.