But the self-destruct function, which should destroy the cells at a predetermined time or when they left their intended environment, has rarely been tested outside of a lab. There’s no way to limit a cell’s interaction with other cells or with its environment. There’s no guarantee that the cells won’t mutate or replicate incorrectly; synthetic skin cells, for example, could reproduce out of control, causing a cancer-like tumor.

Nor is there any way to ensure that these synthetic organisms won’t share genetic material with organisms in the environment, giving rise to who knows what kind of microbes. “Engineers hope that they can engineer organisms that are going to behave in a way they predict,” says David J.J. Gresham, an assistant professor of biology at New York University. “But as soon as they put it into some selective environment, evolution is going to take over.”