Last fall, Chinese scientist He Jiankui claimed to have edited the genes of two newborn twin babies. As reported late last year, He recruited couples for an “AIDS vaccine development project” at a university in Shenzhen, China. After creating embryos from the eggs and sperm of the couples, He used a technique called CRISPR to attempt to delete part of a gene named CCR5. Scientists have discovered that a broken CCR5 gene conveys resistance to HIV infection, and so in theory intentionally damaging the gene could provide the same resistance. However, the use of CRISPR on human embryos that will be born is untested and almost certainly unsafe, which is why He’s work was immediately condemned by scientists and ethicists around the world.