What is
Prader-Willi Syndrome?

A clear explanation of PWS symptoms, causes, diagnosis, genetics, treatments, and research.

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Prader-Willi syndrome (PWS) is a rare, complex genetic disorder that affects many systems in the body and is present from birth. It is caused by missing or inactive genetic material in a specific region of chromosome 15.

PWS occurs in approximately 1 in 15,000 births and affects people of all sexes, races, and ethnicities.

PWS is best known for hyperphagia, a chronic and intense drive to eat that does not turn off. But PWS affects much more than appetite. It can influence muscle tone, growth, hormones, metabolism, learning, behavior, sleep, temperature regulation, and other aspects of health.

The symptoms of PWS vary widely from person to person and can change throughout life.

Young girl in a purple shirt and blue shorts sitting on a bench outdoors. PWS can affect each person differently. Not every person will experience every feature, and the severity of symptoms can vary considerably.

Muscle tone

Low muscle tone, called hypotonia, is one of the earliest signs of PWS. Strength generally improves over time, but motor milestones may be delayed. Balance, coordination, endurance, and physical development may also be affected.

Feeding, appetite, and metabolism

Babies with PWS often have difficulty sucking and feeding, and some require tube feeding or additional nutritional support. Later, many people with PWS develop hyperphagia, an intense drive to eat, along with easy weight gain and differences in metabolism.

Growth and hormones

PWS can affect the body’s hormone systems. Common concerns include growth hormone deficiency, short stature, reduced muscle mass, increased body fat, incomplete sexual development, and infertility.

Learning and development

Developmental delays and learning challenges are common, but abilities vary widely. Some individuals may also experience speech, language, or motor-planning difficulties.

Behavior and emotional regulation

PWS may affect emotional and behavioral regulation. Challenges can include anxiety, difficulty managing strong emotions, repetitive or obsessive-compulsive behaviors, and skin picking.

Sleep and other body systems

Sleep disturbances, excessive daytime sleepiness, temperature regulation issues, a high pain threshold, and other medical concerns may also be associated with PWS.

PWS can look very different from infancy through adulthood, and symptoms and support needs change over time. Researchers have also identified a series of nutritional phases that describe how feeding, appetite, metabolism, and weight may evolve.

Early Life

Newborn baby resting in a hospital bassinet. Babies with PWS commonly have very low muscle tone, a weak cry, and difficulty sucking or feeding. Some are unable to breastfeed and may need tube feeding or other support to receive adequate nutrition.

As babies grow, strength and muscle tone generally improve. Motor milestones are often reached later than expected.

Childhood and Adolescents

Young child wearing a riding helmet and glasses standing in an indoor horse arena. Changes in metabolism and weight may begin before a noticeable increase in appetite. Over time, many individuals develop an increased interest in food and, eventually, hyperphagia. The timing and intensity of these changes can vary significantly.

Learning, hormonal, physical, behavioral, emotional, and sleep-related needs may also evolve as a child grows. The right support will look different for each person and may change over time.

Adulthood

Young man wearing glasses and a blue shirt smiling indoors. Adults with PWS can continue to build skills, relationships, routines, and meaningful roles in their communities. Many also need ongoing support with food security, physical and mental health, hormones, sleep, behavior, and daily living. The type and level of support vary from person to person.

Because adults with PWS have an increased risk for mental illness, significant changes in mood, behavior, sleep, or day-to-day functioning should be taken seriously and evaluated.

PWS is caused by a lack of active genetic material in a specific region of chromosome 15, known as 15q11-q13.

People typically inherit one copy of chromosome 15 from each parent. Certain genes in the PWS region are normally active only on the copy inherited from the father. PWS occurs when those genes are missing or inactive.

This can happen in three primary ways: Diagram showing three genetic causes of Prader-Willi syndrome: paternal chromosome 15 deletion, maternal uniparental disomy (UPD), and an imprinting defect.

PWS by Deletion

Most often, part of the chromosome 15 that was inherited from the person’s father is missing, or deleted, in this critical region. This small deletion occurs in approximately 60% of cases and usually is not detectable with routine genetic analysis such as amniocentesis.

PWS by UPD

Another 35-40% of cases occur when an individual inherits two chromosome 15s from their mother and none from their father. This scenario is termed maternal uniparental disomy (UPD).

PWS by Imprinting Defect

Finally, in a very small percentage of cases (1-3%), a small mutation in the Prader-Willi region causes the paternal chromosome 15 genetic material (although present) to be inactive.

Frequently Asked Questions

How is PWS diagnosed?

PWS is diagnosed through genetic testing, typically using a blood test called DNA methylation analysis.

Methylation analysis can identify the genetic patterns associated with all major forms of PWS. Additional testing may then be needed to determine whether PWS was caused by a deletion, maternal UPD, or an imprinting defect.

A FISH test can identify PWS caused by a deletion, but it cannot identify every form of PWS. For that reason, methylation analysis is generally the preferred test for diagnosis.

Is Prader-Willi syndrome hereditary?

Most cases of PWS caused by deletion or maternal UPD occur randomly and are not associated with a significantly increased chance of PWS occurring in a future pregnancy.

Some imprinting defects can be inherited and may carry a higher chance of recurrence within a family.

Families with questions about their genetic subtype or the likelihood of PWS occurring again should speak with a genetic counselor.

Do symptoms differ by genetic subtype?

There may be subtle differences associated with the genetic subtypes of PWS.

For example, people with a deletion may be more likely to have lighter skin and hair than other family members and may have a higher likelihood of seizures. People with maternal UPD may have an increased likelihood of autism spectrum disorder or certain mental health concerns.

However, there is considerable overlap between the subtypes. Symptoms, strengths, abilities, and support needs can vary widely, even among people with the same subtype.

Is there a cure for Prader-Willi syndrome?

Currently, there is no cure for PWS.

Most research to date has been targeted towards treating specific symptoms (see Diagnosis & Treatments).

For many individuals affected by the disorder, the elimination of the most difficult aspects of the syndrome—the insatiable appetite and anxiety—would represent a significant improvement in quality of life and the ability to live independently.

Why does Prader-Willi syndrome affect so many parts of the body?

Many symptoms of PWS are believed to be connected to dysfunction of the hypothalamus, a small part of the brain that helps regulate several important body functions.

These functions include hunger and fullness, body temperature, sleep, fluid balance, emotions, pain, hormones, and fertility.

Researchers are still working to understand exactly how the genetic changes that cause PWS lead to hypothalamic dysfunction and other symptoms.

Why does PWS affect people differently?

PWS can look very different from one person to another. Genetic subtype may contribute to some differences, but it does not fully predict which symptoms a person will experience or what support they will need.

The PWS region of chromosome 15 is one of the most complex regions of the human genome, and researchers do not yet fully understand how the loss of activity in this region leads to the wide range of symptoms associated with PWS. Natural variation in the thousands of genes outside the PWS region may also contribute to differences among individuals.

Scientists continue to study these factors to better understand why PWS affects each person differently.

How is FPWR advancing progress for PWS?

The Foundation for Prader-Willi Research advances research and therapeutic development to address the most significant challenges of PWS.

FPWR funds promising science, builds the tools and data researchers need, engages industry partners, and helps prepare the PWS community for successful clinical trials. We support research focused on improving care today, developing meaningful treatments, and exploring genetic therapy approaches.

Together with families, researchers, clinicians, and industry partners, we are turning scientific discoveries into new possibilities for people with PWS.

See How FPWR Moves Research Forward

Where does the name Prader-Willi come from?

Prader-Willi syndrome is named for Swiss physicians Andrea Prader, Alexis Labhart, and Heinrich Willi, who described the condition in 1956 after identifying a shared pattern of symptoms in nine children.

Older medical literature may refer to the condition as Prader-Labhart-Willi syndrome or Prader-Willi-Fanconi syndrome, but Prader-Willi syndrome, or PWS, is the name most commonly used today.

Prader-Willi syndrome is also sometimes misspelled as "Prada Willi" syndrome, "Prader Labhart Willy," or "Prader Willy" syndrome.

Where can I learn more about Prader-Willi syndrome?

Throughout our website, you can find:

Plain-language information about PWS and its genetics
Research and clinical trial updates
Educational webinars and conference presentations
Resources for families and caregivers
Opportunities to participate in research

If you'd like to explore more in-depth medical or scientific information, these trusted resources can be helpful:

MedlinePlus – Easy-to-understand information about PWS, genetics, and inherited conditions.
GeneReviews® – Comprehensive clinical information intended primarily for healthcare professionals.
PubMed – A searchable database of published biomedical research for those interested in reading original scientific studies.
National Organization for Rare Disorders (NORD) – hosts a rare disease database that can be helpful in researching genetics and other aspecs of PWS.
National Institutes of Health (NIH) – Educational resources on genetics, genomics, and rare disease research.

Because new discoveries are happening rapidly, FPWR continually reviews emerging research and highlights the findings that matter most to the PWS community through our Blog and educational resources—helping translate complex science into information you can use.

What is
Prader-Willi Syndrome?

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