ATM and ataxia telangiectasia: Notably, complete absence of ATM kinase, in patients with ataxia telangiectasia, a cancer-prone neurodegenerative disease, is not lethal.28 The majority of the variants in ATM in patients with classic ataxia telangiectasia are biallelic truncating mutations that result in a total loss of destabilized ATM protein.28 Given the vital role of ATM in the DNA damage response for DNA repair, cell cycle checkpoint activation, and apoptosis,29 the viability associated with loss of ATM in ataxia telangiectasia patients is intriguing.