Rett Syndrome is due to a genetic mutation in MECP2 gene, on the X Chromosome. It occurs almost exclusively in girls - sadly, boys die at birth.
Diagnosis is based on symptoms and can be confirmed with genetic testing, and will typically occur in 1 in 15,000 births. Stage one called early onset typically begins 6-18 months of age, and at stage two, the child loses purposeful hand skills, speech and mobility.
There is currently no known cure for Rett Syndrome.