Symptoms of mental retardation and autism have been reversed for the first time in laboratory mice.
US scientists created mice that showed symptoms of Fragile X Syndrome - a leading cause of mental retardation and autism in humans.
They then reversed symptoms of the condition by inhibiting the action of an enzyme in the brain.
The study, by Massachusetts Institute of Technology, appears in Proceedings of the National Academy of Sciences.
Fragile X Syndrome is linked to mutation in a gene carried on the X chromosome called FMR1.
It can cause symptoms ranging from mild learning disabilities to severe autism.
The researchers, based at MIT's Picower Institute for Learning and Memory, targeted an enzyme called PAK which affects the number, size and shape of connection between brain cells.
They found that inhibiting the enzyme stopped mice with Fragile X Syndrome behaving in erratic ways.
Prior to treatment they showed signs of hyperactivity, purposeless and repetitive movements.
Further analysis showed that not only were structural abnormalities in connections between brain cells righted, proper electrical communication was restored between the cells.
In the brain small protrusions called dendritic spines are responsible for communication between cells.
People with Fragile X Syndrome have more dendritic spines than usual, but each is longer and thinner, and transmits weaker electric signals.
Blocking PAK activity in the lab mice corrected these abnormalities.
Researcher Dr Susumu Tonegawa stressed that the mice were not treated until a few weeks after symptoms of disease first appeared.
"This implies that future treatment may still be effective even after symptoms are already pronounced," he said.
Professor Eric Klann, of New York University's Center for Neural Science, agreed that the research was potentially significant.
He said: "This is very exciting because it suggests that PAK inhibitors could be used for therapeutic purposes to reverse already established mental impairments in fragile X children."