Activation of ATM kinase in response to DNA damage in the form of DNA double-strand breaks (DSBs) 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). ATM dimer exists in a preformed complex with KAT5 (Tip60) histone acetyltransferase (Sun et al. 2005).