Scientists say they are closer to finding new ways to treat a disease affecting the brain that is passed from parents to children.
The culprit gene sits on the X chromosome
People with Fragile X syndrome lack a healthy gene on the X 'sex' chromosome that means they have mental impairment.
Few medications exist that help, but the University of Pennsylvania team have been able to test new drugs on specially modified fruit flies.
The flies were bred to have the same mutant gene as humans with Fragile X.
The disorder is passed on within families in what is known as X-linked recessive pattern of inheritance.
Women tend to be carriers of the genetic defect because their other X chromosome may be healthy and compensate. But they can pass the damaged X on to some of their children.
Any son who inherits the damaged X will be affected, while daughters may only be carriers.
The main problem in Fragile X is mental impairment. This may range from a normal IQ to severe learning disabilities.
Other symptoms include hyperactivity, attention deficit disorder, emotional and behavioural problems, anxiety, mood swings and seizures.
Many children with Fragile X are mistakenly labelled as simply badly behaved or as having autism.
There is no cure, but behavioural therapies can help. Families can also seek genetic screening.
Dr Thomas Jongens and colleagues had already found mouse and fruit fly models of Fragile X showed similar symptoms to humans with Fragile X.
They also had a tendency to poor communication between nerve cells due to increased activity of a receptor found on nerve cells called mGluR.
Dr Jongens' team reasoned that mGluR overactivity might be the root of many of the cognitive problems related to Fragile X.
They tested whether drugs know to block this receptor, such as lithium, might help.
"What we found was very striking," said Dr Jongens.
The drug restored memory-dependent courtship behaviour in the mutant flies and reversed some of the nerve cell defects.
"Similar modulation of mGluR activity in Fragile X patients should be explored," he told the journal Neuron.
Lynne Zwink, chairwoman of the Fragile X Society, said: "This is important research, but more studies are needed.
"This is not a cure but we are thrilled that this research is going on.
"It has moved our understanding of Fragile X on another step."