Rett syndrome is a neurological and developmental disorder that mostly occurs in females. Infants with Rett syndrome seem to grow and develop normally at first, but then stop developing and even lose skills and abilities. For instance, they stop talking even though they used to say certain words. They lose their ability to walk properly. They stop using their hands to do things and often develop stereotyped hand movements, such as wringing, clapping, or patting their hands. Rett syndrome is considered one of the autism spectrum disorders. Most cases of Rett syndrome are caused by a mutation on the MECP2 gene, which is found on the X chromosome.