Activation of ATM kinase in response to shortened telomeres requires association of ATM dimers with the MRN complex bound to DNA ends. MRN subunit RAD50 is essential for ATM dimer binding (Lee and Paull 2005, Wu et al. 2007). Dissociation of the shelterin complex from telomeres activates ATM (Karlseder et al. 1999), consistent with a mutually exclusive binding of shelterin and MRN to telomeric DNA (Wu et al. 2007).