Immortal bacteria and unstoppable flu viruses may not be imminent, but the potential for Darpa's synthetic DNA to work its way into other organisms is very real. "DNA transfers from one microorganism to another happen all the time," says Roy Curtiss, the director of the Center for Infectious Diseases and Vaccinology at Arizona State University. "Worst-case scenario?" says Todd Kuiken, a researcher from the Synthetic Biology Project, an independent study group. "The kill switch doesn't work, and then these things start mutating and mating and splitting—the normal process of an organism." Or the kill switch could work too well: Imagine if a kill-switch-equipped organism managed to form hybrids with microbes essential to the oceanic food chain, triggering a widespread die-off. The consequences could make an oil spill look good.