The MECP2 gene mutation is most often found in Rett syndrome (RTT). Some females with such mutations may exhibit features more typically seen in autism. While RTT occurs primarily in girls, autism occurs much more frequently in boys. In both conditions, speech and emotional contact are impaired. However, females meeting criteria for RTT do not meet those for autism and symptoms seen in RTT and not in autism include deceleration of the rate of head growth, loss of purposeful hand skills, and mobility or the irregular breathing patterns.
While hand flapping is seen frequently in autism, the repertoire of purposeless hand stereotypes common to RTT are not seen in autism. The girl with RTT almost always prefers people to objects, but the opposite is seen in autism. Unlike those with autism, the RTT girl often enjoys affection. While girls with RTT often have autistic-like features at an early age, these features disappear. [source: http://www.rettsyndrome.org]