ATM and ataxia telangiectasia: Previously, acute telomere attrition was observed in peripheral blood lymphocytes from AT patients.[11] In this respect, it was suggested that AT cells are constitutively in a state of oxidative stress, which might explain why enhanced telomere loss with each cell division occurs accompanied by the appearance of chromosome end-to-end fusions and extra chromosomal telomeric fragments.[8] Findings are indicative that ATM may be at the apex in activating defense mechanisms against oxidative stress.