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.

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