The study in the journal Science Translational Medicine was done on lab mice, so any potential application for humans remains far off, cautioned the authors.

But the findings point to a pathway for further research on an inherited disability that has no cure and affects about one in 4,000 males and one in 8,000 females.

Those with fragile X syndrome display a range of cognitive problems and learning disabilities, may have unusually long faces and large ears, and about 30 per cent are also on the autism spectrum.

"We are a long way from declaring a cure for fragile X, but these results are promising," said lead author Xinyu Zhao, a professor of neuroscience at the University of Wisconsin-Madison.

The drug, known as Nutlin-3, is currently in phase 1 trials for the eye cancer retinoblastoma, and has not been approved for widespread use.

Researchers found when they gave the drug to mice, it could reverse damage from a genetic mutation that causes fragile X syndrome.

This mutation means mice fail to make a protein known as FMRP, which prevents the formation of new neurons, so they cannot remember things, like a toy or object they might have recently been checking out.

When mice with this defect were given Nutlin-3 for two weeks, they "regained the ability to remember what they had seen -- and smelled -- in their first visit to a test chamber," said the study.

A small dose of the drug -- about 10 per cent the amount being tested currently in human trials -- appeared to block the last stage of the chain reaction set off by a mutation in the FMRP gene.

"There are many hurdles," said Zhao. "Among the many questions that need to be answered is how often the treatment would be needed. Still, we've drawn back the curtain on fragile X a bit, and that makes me optimistic."