Rett syndrome is a disorder of the nervous system that leads to developmental reversals, especially in the areas of expressive language and hand use.

Rett syndrome occurs almost exclusively in girls and may be misdiagnosed as autism or cerebral palsy.

Studies have linked many Rett syndrome cases to a defect in the methl-CpG-binding protein 2 (MeCP2) gene. This gene is on the X chromosome. Females have two X chromosomes, so even when one has this significant defect, the other X chromosome is normal enough for the child to survive.

Males born with this defective gene do not have a second X chromosome to make up for the problem. Therefore, the defect usually results in miscarriage, stillbirth, or very early death.

The condition affects about 1 out of 10,000 children. Groups of the disease have appeared within families and certain geographic regions, including Norway, Sweden, and northern Italy.