UK scientists believe they created a groundbreaking form of therapy that could revolutionize the way Parkinson's disease is treated.
One of the patients involved in the gene-therapy trial, Sheila Roy, says it was like turning back the clock 10 years.
She is one of only 15 people worldwide to have had the new treatment, which effectively creates a medicine factory in her brain.
Parkinson's disease occurs when the brain gradually stops producing the nerve-controlling chemical dopamine. Over time, symptoms such as tremors, slow movement and stiffness get worse.
ProSavin, the new treatment, uses a "stripped-down" virus to transport dopamine-making genes into the brain. It is injected into a region called the striatum that helps control movement.
Once the virus gets into the brain cells, it reprograms them to gradually start producing their own dopamine.
Roy was diagnosed with Parkinson's in her forties. After 17 years with the disease, she suffered from severe tremors, and a lack of balance made simple tasks such as writing impossible.
"People would take knives off me in the kitchen because I was everywhere with the knife," she said. "My vocal cords would suddenly shut so I can't breathe."
She added, "If I hit a wall of people, then I can't function -- I just stop -- but I'm starting to see a glimmer of the person I used to be, which is exciting."
Philip Buttery, from the Cambridge Centre for Brain Repair, in eastern England, said it was still early but that the treatment appears to be having positive results.
"It seems to be having an overall beneficial effect in smoothing out people's days, probably allowing a slight dose reduction in medication and, in some patients, a better sleep pattern and a better quality of life for all," he said.
It is not a cure, and more clinical trials are needed, he added.
The scientists at Oxford BioMedica, in southern England, also are developing gene therapy treatments for other degenerative illnesses.
Chief scientific officer Stuart Naylor said, "Rather than popping lots of tablets, the idea of a single shot therapy, single shot treatment, single shot placement of a gene therapy that provides that long-term therapeutic correction is something that hasn't been ever experienced before in medicine."