Biologists worldwide have fallen in love with Crispr for its rapid, efficient gene-editing powers. Now they can swiftly engineer mouse strains with certain defects, letting them study diseases (and explore potential treatments) more easily than ever before. Cures for humans are next. The Crispr startup Editas Medicine expects to launch its first clinical trial, for a congenital eye disease that causes blindness, this year. They’ll load up a virus with tools to snip out the mutated gene, then inject it into a person’s retina. And the US National Institutes of Health has already approved the first wide-scale trial of a Crispr-based cancer treatment: Scientists at the University of Pennsylvania will remove T cells from cancer patients, make three edits, then reintroduce these immune responders back into the body to detect and attack cancer cells. In China, researchers have already made similar tweaks to white blood cells to tackle cancer. But they’re also pushing into dicier territory, using the technique to modify human embryos—and thus potentially future generations. Scientists in the US may be fiddling with similar sorts of heritable modifications in mice but have no plans to do so in people. For now.