Stay on target. That’s the mantra you hear in labs and biotech companies around the world as they snip away at DNA. All the techniques for gene editing—from the famous Crispr-Cas9 to the older TALENs and zinc-finger nucleases—share a problem: Sometimes they don’t work. Which is to say, they have “off-target effects,” changing a gene you don’t want changed or failing to change a gene that you do. And DNA is not something you want poorly rewired.

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