E-Mail IMAGE: CtIP/Ctp1 stimulates endonuclease activity of the MRN complex, which not only polishes "dirty " ends of DNA breaks, but promotes accurate repair of DNA breaks through the homologous recombination mechanism. view more Credit: PNAS Scientists at Tokyo Institute of Technology (Tokyo Tech) have uncovered mechanisms underlying the activation of the MRN complex-- the cell's DNA scissors. Using purified yeast proteins, they demonstrated that phosphorylation of Ctp1, a homolog of a tumor-suppressor protein, plays a key role in activating MRN complex's DNA clipping activity. Intriguingly, a short segment of yeast Ctp1 or its human counterpart could stimulate endonuclease activity of their respective MRN complexes, suggesting its conserved function across species.