ATM and ataxia telangiectasia: Mechanistically speaking, poor cellular growth caused by mutations in a particular gene can be thought of as the main explanation for the growth restricted phenotype, but it has been shown that mutations in other DDR genes that share the cellular phenotype of poor growth, like ATM, do not replicate the compromised growth phenotype at the organism level: growth retardation is absent in ataxia telangiectasia (AT).