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
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.
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.