Genetics Northern California

Fragile X Syndrome

What is Fragile X syndrome (FXS)?
How common is Fragile X?
What causes Fragile X?
How is Fragile X diagnosed?
Can Fragile X be diagnosed during pregnancy?
How is Fragile X treated?
Where can I get more information about Fragile X?

What is Fragile X syndrome?
Fragile X syndrome (FXS) is an inherited condition that causes intellectual disabilities, learning and behavior challenges and recognizable physical features. The severity of the condition can vary, even in the same family. Males are more likely to have FXS and often have more a severe course than females with the condition.

Medical concerns in FXS
Fragile X syndrome has the most impact on learning, but there are a few medical concerns more common in people with this condition including: 
  - Heart conditions (mitral valve prolapse; enlarged aortic root)
  - Connective tissue problems (loose joints; flat feet)
  - Seizures

Learning and behavior in FXS
There is a wide range of learning abilities in individuals with FXS. Learning is usually delayed, and can vary from mild delays to more significant delays. Most males with FXS have moderate to severe intellectual disabilities. There is a much wider range in females; from normal development to significant delays. Behavior concerns are common and may include autistic behaviors, hyperactivity, social anxiety, and an increased risk for aggression. Despite these concerns, parents report their children as friendly and thoughtful, with strong memory skills, and a sense of humor.

How common is FXS?
It is estimated that one in every 4,000 males and one in every 8,000 females has FXS. The features of this condition are extremely variable, so the diagnosis may be missed in newborns. Both males and females can have FXS.

What causes FXS?
Fragile X syndrome is caused by a change (mutation) in a gene called FMR1. The FMR1 gene codes for a protein that is important for brain health. Individuals with FXS have low levels of the FMR1 protein.

FXS is inherited as an X-linked condition. That means the FMR1 gene is located on the X chromosome. Males have only one X chromosome, while females have two X chromosomes. This is why FXS affects males and females differently. However, the inheritance of FXS is complicated. This condition can affect a person regardless of their sex.  And, both males and females who do not have FXS can silently carry changes in the FMR1 gene, called pre-mutations.  

How is FXS diagnosed?
The diagnosis may be suspected when a person has features commonly seen in FXS.  The diagnosis is confirmed by a blood test that studies the FMR1 gene. 

To learn more about genetic testing for FXS: Fragile X Testing

Can FXS be diagnosed during pregnancy?
FXS can be diagnosed during pregnancy by testing a DNA sample from the developing baby. The sample can be obtained either by chorionic villus sampling (CVS) or amniocentesis. Special genetic testing is needed to study the FMR1 gene. This study is not part of routine prenatal testing. Prenatal testing for FXS is offered when a pregnant woman is known to carry a mutation in the FMR1 gene.

How is FXS treated?
Health care for children and adults with FXS is arranged based on symptoms. Children may benefit from early intervention programs to support learning, and routine medical visits to watch for related medical problems. Treatment may also include medication to help with behavior concerns. There are medications being studied that may improve learning, but there are no proven therapies at this time.

Where can I get more information about Fragile X?
Genetics Home Reference: Fragile X syndrome - General information about FXS provided as a service of the U.S. National Library of Health. Also includes links to many related sites.

For more information about Fragile X syndrome visit the National Fragile X Foundation website.

Last reviewed: January 5, 2018

Reviewed by: Kimberly Barr, MS, LCGC