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.
This implies that future treatment may still be effective even after symptoms are already pronounced
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.