Genetic engineering can correct the worst symptoms in mice of fragile X syndrome, the most common inherited cause of mental retardation and autism, US researchers report.

They say a drug may do the same thing as their gene tinkering, perhaps providing a treatment for fragile X syndrome and other causes of retardation and autism, too.

Fragile X causes seizures, impaired memory, learning disabilities, hyperactivity, severe mental retardation and accelerated body growth.

It is caused by a mutation in a gene on the X chromosome that prevents activation, or expression, of a protein called fragile X mental retardation protein or FMRP.

There is no treatment.

Professor Mark Bear, from the Howard Hughes Medical Institute at the Massachusetts Institute of Technology, and colleagues created this mutation in mice, and corrected their symptoms by adding a few more genetic tweaks.

Writing in the journal Neuron, they say an experimental class of drugs could have the same effect.