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.

“This is now the most powerful system we have in biology,” says Sabatini. “Any biological process we care about now, we can get the comprehensive set of genes that underlie that process. That was not possible before. There was no way one could imagine doing that.”

CRISPR involves two components—an enzyme called Cas9 that slices DNA, and a guide molecule that deploys Cas9 to the right target. Moffat and Sabatini’s teams both created libraries of guides, targeted to the majority of human genes. Both used the libraries to unleash Cas9 upon individual genes, to see if their loss would affect a cell’s ability to grow or reproduce. And both computed a kind of essentiality score to measure how critical the various genes are.

The two teams identified between 1,600 and 1,800 genes as being essential—around one in ten of the ones they analyzed. “It's a big treasure chest of results,” says Moffat.

Compared to their more dispensable peers, these genes are more strongly activated, and unlikely to carry disabling mutations. They’re more likely to have similar (and equally indispensable) counterparts in other species. And as Moffat puts it: “Essential genes like to hang out with each other.” That is, they build proteins that unite to form large collaborative complexes.

Predictably, the essential genes tend to be involved in fundamental biological processes, like copying DNA, transcribing genes, and building proteins. Perhaps less predictably, Sabatini found that we have no idea what 18 percent of the essential genes on his list are doing—a testament to how much we still have to learn about the most critical parts of ourselves.