Rett Syndrome (RTT)is a very complex thing and the the National Institute of Neurological Disorders and Stroke has a great fact sheet that you can see by following this link. In the United States there are two large organizations that are funding research for RTT and they also have comprehensive information on RTT. You can visit RettSyndrome.org or the Rett Syndrome Research Trust to see how they explain it.
As an organization that is dedicated to family support, this is how we explain it.
-is genetic but it doesn’t run in families
-affects the ability to do what you want when you want to
-current treatments are limited to fixing the problems that it causes, like a feeding tube or seizure medication
-is being studied in multiple clinical trials and we have never been closer to help for those affected