The symptoms of a severe brain disorder similar to autism, which affects around 10,000 UK children, could be reversed, scientists believe.
A team at Edinburgh University made symptoms of Rett syndrome disappear in mice by activating a single gene, the magazine Science reports.
The condition, which mainly affects girls, was previously thought to be irrevocable.
It can leave children mute and with multiple disabilities.
Rett syndrome is a complex neurological disorder. Although present at birth, it becomes more evident during the second year of a child's life.
Many have difficulty walking and have breathing irregularities including hyperventilation, breath-holding and air swallowing.
The scientists hope their discovery will encourage further research aimed at discovering a treatment for the syndrome.
The research focused on the behaviour of the MECP2 gene that causes the condition to develop.
When the gene was activated in mice which had been born with MECP2 switched off, disease symptoms such as breathing and mobility problems stopped.
Within a few weeks the affected mice were largely indistinguishable from their healthy counterparts.
Researcher Dr Stuart Cobb said: "This breakthrough provides a small piece of hope for sufferers.
"It had been thought the disorder was caused by abnormal development - but activating the MECP2 gene, even in adulthood, proved to be effective in reversing neurological symptoms.
"Analysing the communication between nerves in the brain, we discovered that abnormal signalling is also rectified when MECP2 gene is switched back on.
"The results really were quite astonishing and completely unexpected."
The scientists are hopeful that future therapies could override the faulty MECP2 or counter the action of proteins abnormally expressed by the culprit gene.
Chris James, director of Rett Syndrome Association UK which helped fund the study, said: "This is a very significant step on the road for future therapeutic approaches to Rett syndrome and, whilst the work in this area is still at an early stage, it will give hope to those families affected."