Enter the revolutionary gene-editing technique called CRISPR, which was invented billions of years ago by bacteria, as part of a defense system for hacking into the genes of invading viruses. But in the last five years, scientists have repurposed it into a tool that can edit any gene at will. Unlike RNA interference, which disables the messages transcribed from genes, CRISPR alters the gene itself. It’s like changing the actual text of the book, rather than just gagging someone who’s reading from it. And these changes are very specific—CRISPR rarely misses its target.Two different groups, one led by Moffat and the other by David Sabatini at the Whitehead Institute for Biomedical Research, have used CRISPR to systematically inactivate almost every gene in human cells in a bid to identify all the essential ones.Though their studies differ slightly in their details and results, they both illustrate the true power of CRISPR. Amid much hand-wringing about far-flung and grandiose applications, like creating designer babies or engineering malaria-fighting mosquitoes, scientists are already using this technique to revolutionize our basic understanding of the living world. In a single experiment, researchers can quickly do what their predecessors took years to accomplish.