What is PWS?

By Suzanne Cassidy, MD

A syndrome is a group of signs and symptoms that occur together and characterize a disease or disorder.

Prader-Willi syndrome (“PWS“) was first described in 1956 by Swiss doctors Prader, Labhart, and Willi.  PWS is believed to be among the ten most common syndromes seen in birth defect clinics, and is the most common genetic syndrome causing morbid obesity in children. It exists throughout the world, but most people, including many physicians, are not familiar with this potentially life threatening, devastating, life-long disorder.

Multiple studies in several countries in the developed world suggest that at least one in 15,000 to 30,000 people have the syndrome, which means that about 200,000 people worldwide are affected. In emerging countries very few patients are diagnosed. The following is a simplified description of the cause, manifestations and management of PWS. Additional details are available through IPWSO if desired.



Prader-Willi syndrome is a genetic condition whose cause is quite complex. It occurs when there is lack of expression of one or more genes in a specific region on the chromosome 15 that was inherited from the father, the precise location of the abnormality being designated as 15(q11-q13).


There are 3 common ways that this lack of expression can occur:


1) there is a missing segment at this location from the chromosome 15 that was inherited from the father, called a deletion (found in about 70%);


2) the entire chromosome 15 contributed by the father is missing and instead there are two chromosome 15’s inherited from the mother, which is called maternal uniparental disomy 15, or UPD 15 (found in about 25%), or


3) there is a defect in the normal expression of the relevant genes due to an abnormality in control of gene expression, called an imprinting defect (found in 5% or less). The syndrome generally occurs as an isolated event in the family and risk is low that parents with one affected child will have another child with the same condition in the vast majority of families. There is nothing that either parent did or did not do, either before or during the pregnancy, that causes this genetic alteration.



Prader-Willi syndrome is usually suspected on the basis of the clinical signs and symptoms (see below) and confirmed by genetic testing. Clinical diagnostic criteria have been published in the medical literature and confirmed as accurate. However, because other disorders can have overlapping manifestations, the diagnosis should be confirmed by genetic testing, which is clinically available in many countries and is very accurate. Currently, the best test for Prader-Willi syndrome is called methylation analysis, and it is the best because it will be positive regardless of the genetic type. Other genetic tests can be used to determine the specific type, if that is desired, but it is not necessary to confirm the diagnosis.

Currently, IPWSO offers to cover the deeply discounted cost of genetic testing offered at the Baschirotto Institute for Rare Disorders in Italy for those living in countries where it is not available.

What is PWS: Keep Reading


The pattern of characteristics associated with PWS is consistent among those affected by it, although there is significant variability in severity and not all affected individuals have every characteristic...>


There is often a characteristic facial appearance with a narrow forehead, almond-shaped eyes, and a thin upper lip with downturned mouth. Scoliosis (S-shaped curvature of the spine) may develop. Many individuals have fairer coloring of hair, eyes and skin than other family members...>


There is currently no cure for Prader-Willi syndrome, nor is there a drug that can be prescribed to alleviate all of the symptoms. As of this writing, there is no known substance that has been demonstrated to lessen the drive to eat, although much research is being conducted to find such a substance...>

Not found what you're looking for?

Get in touch and let us know how we can help you with any enquiry about Prader-Willi syndrome and how our organisation works.

Get our email updates



Keep up to date with the latest news, stories and research from the PWS community.


Salisbury House
Station Road
Cambridge CB1 2LA

  • Instagram
  • LinkedIn
  • Facebook
  • Twitter
  • YouTube

Registered as a charity in England & Wales, charity no. 1182873