This may grow into something big. Researchers at Guangzhou’s Sun Yat-sen University say they’ve genetically modified DNA sequences in human embryos for the first time in history, replacing segments that could trigger disease. But the work revealed that splicing isn’t terribly accurate, and thus unfit for “viable” embryos. Some think editing holds promise, but scientists warn that the technology is still too “immature” for human use. While it’s destined to remain a theoretical possibility for now, genome-modification is gestating into a robust ethical debate.