ATM and Ataxia: Formation of this functional, six-protein shelterin complex at telomeres protects telomeres and promotes genome stability by suppressing ATM (Ataxia telangiectasia mutated)- and ATR (ATM and Rad3-related)-dependent pathways, homologous recombination, and c-NHEJ (classical non-homologous end joining) at chromosome ends (Fig. 2).