This condition is caused by missing genes at the very tip of the q arm of chromosome 22. One of the crucial genes that is typically absent and thought to cause most of the symptoms is the Shank3 / ProSAP2 gene.
The condition is variable, but most children with have cognitive delays to some degree, especially with the development of speech. Many will also have low muscle tone resulting in physical delays, problems with eating, sleeping, and behaviour.
Here is an excellent article on the 22q13.3 Deletion Syndrome (Phelan-McDermid Syndrome - by K. Phelan and H. McDermid.
Most of the best information you will find will be at the
Phelan McDermid-Syndrome
Foundation website. The group is very active and holds
informative conferences bi-annually.
Affiliate member, Canadian Organization for Rare Disorders