At the turn of the century, gene therapy had become a cautionary tale for modern medicine. Once hailed as a potential cure for many diseases, the idea of using viruses to patch faulty genes with working DNA had fallen into disrepute, as studies showed that this was not only extremely difficult, but also potentially dangerous. In 1999, Jesse Gelsinger, an 18-year-old with liver disease, died during a trial at the University of Pennsylvania. Three years later, an Anglo-French study of…