Keeping it real: MRX-Sae2 clipping of natural substrates.

The yeast Mre11-Rad50-Xrs2 (MRX) complex and Sae2 function together to initiate DNA end resection, an essential early step in homology-dependent repair of DNA double-strand breaks (DSBs). In this issue of , Wang and colleagues (pp. 2331-2336) and Reginato and colleagues (pp. 2325-2330) report that a variety of physiological protein blocks, including Ku, RPA, and nucleosomes, stimulate MRX-Sae2 endonuclease cleavage in vitro. These studies have important implications for how cells deal with a range of barriers to end resection and highlight the crucial role of Sae2 in activating MRX cleavage at the correct cell cycle stage.