Josiah Zayner, 36, recently made headlines by becoming the first person to use the revolutionary gene-editing tool Crispr to try to change their own genes. Part way through a talk on genetic engineering, Zayner pulled out a syringe apparently containing DNA and other chemicals designed to trigger a genetic change in his cells associated with dramatically increased muscle mass. He injected the DIY gene therapy into his left arm, live-streaming the procedure on the internet. The former Nasa biochemist, based in California, has become a leading figure in the growing “biohacker” movement, which involves loose collectives of scientists, engineers, artists,...