Zellweger syndrome is extremely rare, affecting roughly 1 in 50,000 live births in the United States. That translates to about 80 babies born with some form of the condition each year in the U.S. However, the rate varies dramatically by region, from as rare as 1 in 500,000 in Japan to as common as 1 in 12,000 in one part of Quebec, Canada.
Incidence Across Regions
Zellweger syndrome is part of a broader group called Zellweger spectrum disorder (ZSD), which ranges from severe to mild. The severe end of that spectrum is what most people mean when they refer to Zellweger syndrome specifically. Across all severities, the estimated incidence is about 1 in 50,000 to 1 in 100,000 live births worldwide.
The most striking outlier is the Saguenay-Lac-Saint-Jean region of Quebec, where the incidence reaches 1 in 12,191 live births, the highest reported anywhere in the world. This is traced to a founder mutation: a single genetic change that became unusually common in the French-Canadian population of that area due to historical isolation and a small founding population. In that region, roughly 1 in 55 people carries the relevant gene variant. Japan sits at the opposite extreme, with an incidence of about 1 in 500,000.
Many Mild Cases May Go Undiagnosed
A 2025 population genetics study modeled how many people with ZSD actually exist across seven countries (the U.S., U.K., Germany, France, Italy, Spain, and Japan). Looking only at known disease-causing variants, the researchers estimated nearly 500 living patients across those countries. But when they factored in predicted harmful variants, the numbers jumped considerably: an additional 260 patients with intermediate symptoms and 930 with mild symptoms, all under age 30.
The takeaway is that a significant number of people with milder forms of ZSD likely go unrecognized by current diagnostic practices. The mildest form, sometimes called infantile Refsum disease, can present with subtler symptoms that may not prompt the specific testing needed for a diagnosis. This means the true prevalence of the full spectrum is probably higher than published incidence figures suggest.
Carrier Frequency in the General Population
Because ZSD is autosomal recessive, a child must inherit a faulty copy of the same gene from both parents to develop the condition. Carriers, people with just one faulty copy, have no symptoms. A large-scale analysis of over 60,000 people estimated the carrier frequency for the most commonly affected gene at roughly 1 in 100 to 1 in 530 people, depending on how strictly the researchers defined a disease-causing variant. Even using the most conservative estimate, carriers are far more common than the disease itself, which is typical for rare recessive conditions.
About 60% of all ZSD cases are caused by changes in one specific gene, and another 15% by a second gene. The remaining quarter of cases involve mutations spread across more than a dozen other genes, all of which play a role in building the same cellular structures called peroxisomes.
What Zellweger Syndrome Does
Peroxisomes are tiny compartments inside cells that break down certain fats and perform other metabolic tasks. In Zellweger syndrome, these compartments either don’t form properly or don’t function, which causes very long chain fatty acids and other substances to build up in the body. This accumulation damages the brain, liver, kidneys, and other organs during fetal development and after birth.
Babies with the severe form typically show signs at birth or within the first days of life. These include weak muscle tone, difficulty feeding, seizures, distinctive facial features, and enlargement of the liver. Vision and hearing problems are common. The prognosis for the classic severe form is poor: most infants do not survive beyond their first year, with organ failure being the primary cause of death. Children with intermediate or mild forms of the spectrum can survive into childhood or adulthood, though they often face developmental delays, hearing loss, and progressive neurological problems.
How It Gets Detected
Zellweger spectrum disorder is included in the U.S. Recommended Uniform Screening Panel for newborns, meaning babies born in states that have adopted this recommendation can be screened shortly after birth. Screening works by detecting elevated levels of very long chain fatty acids in a blood sample. If the screen is positive, confirmatory testing measures specific fatty acid ratios in the blood and genetic testing identifies which gene is affected.
For families with a known history of ZSD, carrier testing and prenatal diagnosis are available. Given that carrier rates may be as high as 1 in 100 in the general population for the broadest definition, genetic counseling can be particularly useful for couples who have already had an affected child or who come from higher-risk populations like the Saguenay-Lac-Saint-Jean region.