Knowing how Cas9 works in such magnificent molecular detail matters because while the system is good for editing genes—even very good—it is not perfect. Sometimes it cuts the wrong stretch of DNA. Sometimes it doesn’t cut the stretch it is supposed to. Insights from the new study could lead to “more efficient design of Cas9 mutant with high specificity,” says Osamu Nureki, a biologist at the University of Tokyo who has also worked on the structure of Cas9.