Making the cut.

The genome-editing method called CRISPR has matured into a molecular marvel that much of the world—not just biologists—has noticed, which is why it has been selected Science 9s 2015 Breakthrough of the Year. CRISPR has appeared in Breakthrough sections twice before, in 2012 and 2013, each time as a runner-up in combination with other genome-editing techniques. But this is the year it broke away from the pack, revealing its true power in a series of spectacular achievements. Two striking examples—the creation of a long-sought "gene drive" that could eliminate pests or the diseases they carry, and the first deliberate editing of the DNA of human embryos—debuted to headlines and concern. Each announcement roiled the science policy world. The embryo work (done in China with nonviable embryos from a fertility clinic) even prompted an international summit this month to discuss human gene editing. The summit confronted a fraught—and newly plausible— prospect: altering human sperm, eggs, or early embryos to correct disease genes or offer "enhancements." As a genetic counselor quipped during the discussion: "When we couldn9t do it, it was easy to say we shouldn9t."