Researchers have discovered a way to program cells to inhibit CRISPR-Cas9 activity. “Anti-CRISPR” proteins had previously been isolated from viruses that infect bacteria, but now University of Toronto and University of Massachusetts Medical School scientists report three families of proteins that turn off CRISPR systems specifically used for gene editing. The work, which appears December 15 in Cell, offers a new strategy to prevent CRISPR-Cas9 technology from making unwanted changes.