Gene therapy has been shown to have "significant" clinical benefits for people with Parkinson's disease, according to a study.
Parkinson's is a degenerative condition that effects the brain
The small-scale trial showed the 12 patients' symptoms improved by up to 65% after a year, with no ill effects.
The trial was carried out by biotech company Neurologix Inc, based in the United States.
Steve Ford, chief executive of the Parkinson's Disease Society, described the results as "encouraging".
The trials are the first attempt at using this particular approach to tackle Parkinson's.
The new therapy uses a harmless virus to carry a gene called GAD directly into neurons in a region of the brain called the subthalamic nucleus (STN).
It produces a molecule called GABA, which is normally released by nerve cells to inhibit or dampen activity so that the tremors and other symptoms are reduced.
After a year of the trial, all the patients demonstrated a clinical improvement of at least 25%, judged according to a clinically recognised measure.
Nine patients showed an average improvement of 37%, and five of these demonstrated substantial recovery of between 40% and 65%.
Dr Matthew During, who led the trial, said: "This gene therapy trial is particularly unique and the clinical data unusually promising because the treatment was confined to just one side of the brain."
Mr Ford also said the Parkinson's Disease Society was interested in the results.
"This type of therapy is still at an early stage of development and the phase 1 trial is encouraging in terms of both safety and effectiveness," he said.
"We look forward to further research being undertaken in this area in the future."
About 120,000 people in the UK have Parkinson's, which causes rigidity and uncontrollable tremors.
Currently incurable, it has to be controlled with drugs - but this becomes more difficult as the disease progresses.
People with Parkinson's experience insufficient activity from a brain signalling molecule called dopamine.
They either have too few neurons releasing dopamine, or not enough of the receptors that it stimulates.
Because of this imbalance, the STN becomes highly overactive, which results in Parkinson's symptoms.