Getting therapeutic drugs into the brain is difficult because of a firewall of cells that line the capillaries there. But Alessio Fasano, a gastroenterologist at the University of Maryland, may have found a way through. A few years ago, while studying cholera vaccines, Fasano stumbled across a protein that made victims' intestines dangerously leaky. He later found similar molecules that unlock tightly sealed barricades throughout the body, including the one in the brain. Tests on brain tissue have confirmed that some of these molecules can open the blood-brain barrier, at least briefly. "Many brain tumors are highly susceptible to drugs, but we can't get the drugs across the barrier. Now we have a key," he says.