Engineering DNA damage signaling without DNA damage. (a) The response to DNA double-strand breaks (DSBs) in human cells. The MRN complex localizes to the DNA ends and promotes the recruitment of the protein kinase ATM. ATM phosphorylates histone H2AX to γ-H2AX, which promotes the assembly of MDC1, 53BP1 and RNF8 (not shown) at the site of the DSB. In turn, the recruitment of MDC1 promotes the amplification of the signaling cascade by recruiting additional ATM molecules over many kilobases. Chk1 and Chk2 are the checkpoint kinases that are activated by the DNA damage response and interact with cell-cycle components to cause the arrest of the cell cycle. (b) Tethering of ATM to an array of Lac operator sites (LacO) via the Lac repressor (LacR) promotes the formation of γ-H2AX and recruitment of MRN, MDC1 and 53BP1, followed by activation of Chk1 and Chk2.