The discovery of the gene that controls Rett syndrome could lead to the development of a test to diagnose the disorder before birth and new methods to prevent the debilitating effects of the disease.

The researchers, from the Howard Hughes Medical Institute at Baylor College of Medicine in Houston and Stanford University, published their results in the journal Nature Genetics.

Young girls who have Rett syndrome gradually stop responding to and interacting with their parents at about six to 18 months of age.

Eventually, the girls lose the ability to speak, walk, and use their hands effectively.

Children with Rett syndrome develop repetitive hand motions, such as hand wringing or hand "washing."

They may also develop breathing abnormalities and experience sudden, unexpected heart attacks.

Dr Duane Alexander, director of the National Institute of Child Health and Human Development, said: "Rett syndrome has long frustrated researchers and clinicians because of its unusual pattern of inheritance.

"The discovery is a definite turning point, which we hope will soon lead to better diagnosis, and, ultimately, treatment, for this disorder."

Spontanous occurrence

Rett syndrome affects roughly 1 in every 10,000 females. Except for a few rare families in which more than one child is affected, the disorder occurs spontaneously.

The researchers located the gene for Rett syndrome, called MECP2, on a region of the X chromosome, one of the two sex-determining chromosomes found in human cells.

Normally, the MECP2 gene makes a protein which is needed to switch off a group of other genes.

However, Rett syndrome sufferers have an abnormality in this gene that interferes with the production of the protein.

This means that other genes are not shut down as normal, and therefore over-produce substances that have a negative impact on the nervous system in excessive quantity.

Girls develop the disease gradually because they have two X chromosomes. Only one chromosome carries the mutant gene, and the normal version of MECP2 provides some initial protection against the disease.