The MRN complex consists of the two evolutionarily conserved components (Mre11 and Rad50) and a third less-conserved component (Nbs1/Xrs2). This complex mediates telomere maintenance in addition to a variety of functions in response to DNA double-strand breaks, including homologous recombination, nonhomologous end joining (NHEJ), and activation of DNA damage checkpoints. RAD50 provides ATP-dependent regulatory control of Mre11 nuclease activity by unwinding and/or repositioning DNA ends into the Mre11 active site. (Adapted from FBrf0180055 and PMID:17698079.)