Scientists have created an ‘on’ switch for CRISPR-Cas9 that allows it to be turned on in select cells only, specifically those that have a particular protein-cutting enzyme, or protease. Viruses produce such proteases, as do cancer cells, so the Cas9 variants — called ProCas9 — could be used as sensors for viral infections or cancer. The variants were discovered by circular permutations on wild-type Cas9 designed to produce a stripped-down Cas9 tuned to human cells.