[Microbiologists] stumbled onto tools now known as anti-CRISPRs. These proteins serve as the rocks to CRISPR’s molecular scissors. And soon, they were popping up everywhere: more than 50 anti-CRISPR proteins have now been characterized, each with its own means of blocking the cut-and-paste action of CRISPR systems.

The expansive roster opens up many questions about the archaic arms race between bacteria and the phages that prey on them. But it also provides scientists with a toolkit for keeping gene editing in check.

Some are using these proteins as switches to more finely control the activity of CRISPR systems in gene-editing applications for biotechnology or medicine. Others are testing whether they, or other CRISPR-stopping molecules, could serve as biosecurity counter-measures of last resort, capable of reining in some genome-edited bioweapon or out-of-control gene drive.

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Jennifer Doudna, a biochemist at the University of California, Berkeley, and one of the pioneers of CRISPR gene editing, voices a question that she says is on everyone’s lips: “How do you actually use these in a way that will provide meaningful control?”

“That’s certainly where that whole anti-CRISPR field needs to go,” she says.

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