CRISPR-Cas9 is a revolutionary tool in part because of its versatility: created by bacteria to chew up viruses, it works equally well in human cells to do all sorts of genetic tricks, including cutting and pasting DNA, making pinpoint mutations and activating or inactivating a gene. Adam Arkin and his colleagues, David Savage and Jennifer Doudna, have now made it even more versatile by giving it an “on” switch, allowing users to keep the Cas9 gene editor turned off in all cells except its designated target.
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