Patients with Parkinson's disease often do well for years by taking levodopa-a drug their brain cells turn into dopamine, the neurotransmitter whose decline causes debilitating spasmodic movements. Eventually, though, the drug loses effectiveness, as the patient's brain stops converting it, and symptoms worsen. But gene therapy-in which therapeutic genes supplement or replace missing or damaged ones-may keep the drug working much longer, offering hope that the lives of many Parkinson's patients can be dramatically improved.

Biologists at the University of California, San Francisco, have injected viruses carrying an enzyme-producing gene, called AADC, into monkeys' brains. The enzyme transforms levodopa into dopamine and could keep the drug working in late-stage Parkinson's patients. Krys Bankiewicz, the UCSF gene therapist who is leading the study, says levodopa still controls the symptoms of test monkeys suffering from a severe, chemically induced form of Parkinson's three and a half years after gene transfer. "There are no adverse effects that we can detect, and it's efficacious," he says. Indeed, Bankiewicz is cautiously optimistic that a very small human trial, which would also require injections into the brain, could begin as early as the end of the year.