The ability to edit human genomes may one day transform people’s lives, but in breaking new ground, it would cross a line that has long been controversial. Today, many countries prohibit the creation of genetically altered people, even if the procedure would spare them a devastating and life-shortening disease. The caution comes from the fact that changes to an embryo’s DNA affect not only the child in question, but their sperm or eggs. When the time came, they would pass on the modifications – and any harmful side-effects the procedure may cause – to their children and future generations. The risk of causing unintended harm to unknown numbers of people is only one concern. Inevitably, the ability to tweak the DNA of unborn children raises the prospect of designer babies. There are broader fears too: that eradicating certain mutations could threaten people’s identities, and even the sense of what it means to be human. A more effective way to reduce genetic disease could be a national genetic testing program that identifies people who are carriers of harmful genes and at risk of passing them on.