Researchers using a new technique for editing the genome of living cells have shown that they can cure hemophilia in mice, at least in principle, with a couple of injections that carry out the “cut” and “paste” operations needed to insert a corrective gene. This is the first time this genome-editing technique has succeeded in a live animal. Along with other applications, like two AIDS treatments in preliminary stages, the new technique could be the decisive improvement that gives credibility to the long-struggling field of gene therapy.