Angelman syndrome

Signs and symptoms of Angelman syndrome can overlap with those of autism. Certain behaviors, hand‑flapping, and language problems can be similar to those seen in autism spectrum disorders. This has led to some children with Angelman syndrome being misdiagnosed as having autism. To further complicate things, studies have shown that a small portion of children with Angelman syndrome do in fact also have autism.

If you are interested in learning more on this topic, the following article published by the Angelman Syndrome Foundation offers further details. The discussion of Autism and Angelman syndrome begins on page 17.
http://www.angelman.org/_angelman2/assets/File/facts%20about%20as%202009%203-19-10.pdf