Doctors Kathy Phelan and Heather M.Dermid, are two US researchers who discovered the Phelan-McDermid syndrome in the 90s thanks to new techniques of DNA decomposition, which allowed to see the infinitely small.
They used a technique called the CGH Array that allowed them to see these small deletions on the specific chromosomes causing Phelan-McDermid syndrome. At the time, other simpler methods were only able to detect deletions of a certain size but not the smallest ones. Nowadays, microchip test can be used to detect this syndrome.
Humans have 22 pairs of chromosomes and one additional pair of XY sex chromosomes. And, Phelan-McDermid syndrome is a micro-deletion on chromosome 22 , i.e. a loss of DNA sequence on the end of the long arm (q) of chromosome 22. This loss includes the Shank3 gene (also called PROSAP2) which plays an important role in the connection of neurons in the brain. SHANK3 gene codes for a protein involved in the functioning of the synapses of the cerebral cortex and the cerebellum.

There are three kinds of Phelan-McDermid syndromes:
1. The first type is caused by a loss at the end of the long arm (q) of chromosome 22
2. The second type is caused by loss of both two ends of the long arm which meet in a ring
3. The third types is caused by loss of genetic material on the long arm in addition to some additional genetics from another chromosome (translocation).

Depending on the size of the deletion on chromosome 22 and the exact position of the deletion, we can generally know which genes are missing. On the other hand, the functions of these missing genes are often not yet known.
See list of genes

Characteristics of Phelan-McDermid syndrome:

Every Phelan-McDermid child is different but most have a learning disability.
- Delay in motor development and learning ability.
- Delay or lack of language acquisition
- Decrease or absence of muscle tone
- Growth from normal to fast
- 80% of people with the syndrome have an autism spectrum disorder.
- A permanent chewing or biting habit
-Epileptic seizures can occur with age with sometimes subtle signs.

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Right now, too few people with Phelan-McDermid syndrome are old enough to have a full understanding of its full spectrum.
In the United States, The American Foundation for Phelan-McDermid Syndrome, with whom we work closely and support, is in charge of listing all the people diagnosed on an international registry which eventually will help research. This registry shows that the number of people diagnosed is increasing every year. You can follow the global distribution of people with Phelan-McDermid syndrome by clicking here

* PMS: Phelan-McDermid syndrome