FIGURE 1: Structural organization of the MRX complex. The ATP-bound state of Rad50 nega-tively regulates MRX nuclease activity by masking the Mre11 nuclease sites. ATP hydrolysis by Rad50 causes conformational changes of both Rad50 and Mre11, resulting in disengagement of Rad50 dimer and exposure of the Mre11 active sites that can access DNA to initiate DSB resection. The Mre11 nuclease sites are indicated by yellow stars. ATP and ADP are indicated by purple and pink dots, respectively.