CDC Features

CDC Features

Learn More About Fragile X Syndrome

Photo: three children posing for a photo.

July 22 is National Fragile X Awareness Day. Fragile X syndrome is the most common known cause of intellectual disability that can be inherited. CDC and its partners have been working on several public health activities to find out more about fragile X syndrome.

Fragile X is a group of genetic disorders that can affect individuals and their families in many ways because they are all caused by changes in the same gene, the Fragile X Mental Retardation 1 (FMR1) gene. The group of fragile X conditions includes:

Fragile X syndrome is the most common known cause of intellectual disability that can be inherited.
Fragile X-associated tremor/ataxia syndrome (FXTAS) involves tremors and problems with walking, balance, and memory. FXTAS occurs in some older men and women who carry certain changes in the FMR1 gene. (See people with a premutation.)
Fragile X-associated tremor/ataxia syndrome (FXTAS) can cause tremors and problems with walking, balance, and memory. FXTAS occurs in some older men who have changes in the FMR1 gene.

What is Fragile X Syndrome?

Fragile X syndrome is the most common known cause of intellectual disability, also known as mental retardation, that can be inherited (passed from one generation to the next).

Physical and behavioral signs that a child has fragile X syndrome include:

Not sitting, walking, or talking as early as other children (this is known as having developmental delays)
Learning disabilities
Speech and language delays
Behavioral problems such as attention-deficit/hyperactivity disorder (ADHD)

Children often have a typical facial appearance that gets more noticeable with age. These features include:

A large head
A long face
Prominent ears, chin, and forehead

Males who have fragile X syndrome usually have some degree of intellectual disability that can range from mild to severe. Females with fragile X syndrome can have normal intelligence or some degree of intellectual disability with or without learning disabilities. Autism spectrum disorders also occur more frequently in children with fragile X syndrome.

How Many People Have Fragile X Syndrome?

The exact number of people who have fragile X syndrome is unknown, but it is estimated that about 1 in 4,000 males and 1 in 6,000 to 8,000 females have the disorder. Although fragile X syndrome occurs in both males and females, females usually have milder symptoms.

What Causes Fragile X Syndrome?

See "Specific Topics" on the Fragile X Syndrome page for a video and transcript.

FXS is caused by a change (mutation) in a gene on the X chromosome. Genes contain codes, or recipes, for proteins. Proteins are very important biological components (parts) in all forms of life. The gene on the X chromosome that causes FXS is called the Fragile X Mental Retardation 1 (FMR1) gene. The FMR1 gene makes a protein that is needed for normal brain development. This protein is not made in individuals who have FXS.

Click here to view a video about what causes fragile X syndrome (Note: this video is a large file and may not work well for users with dial-up Internet access. The clip is best viewed using Media Play 9 or RealOne Player. If you have difficulty viewing the clip, please send us an email.) Click here for a transcript of the video.

How is Fragile X Syndrome Diagnosed?

Fragile X syndrome can be diagnosed by testing a person's DNA from a blood sample. A physician or genetic counselor must order the test. Testing can also be done to detect changes in the FMR1 gene that can lead to the different conditions mentioned above.

What is CDC Doing about Fragile X Syndrome?

Since 2005, CDC and its partners have been working on several public health activities to find out more about fragile X syndrome. These projects include:

Fragile X Clinic Consortium
The fragile X community is linking together clinics that care for people with fragile X, FXTAS, and FXPOI. The purpose is to improve care, identify research projects, and develop a patient registry. CDC is providing support for this effort.
Fragile X Pilot Surveillance Projects
CDC is supporting a project to estimate the prevalence of FXS in individuals who have autism and intellectual disability. Researchers at Johns Hopkins University and the Medical University of South Carolina are working on this project.
National Fragile X Survey
CDC is working with researchers at Research Triangle Institute (RTI) on a national survey that will identify the needs of families with fragile X syndrome. A total of 1,221 families have enrolled in the survey. The results from this survey will help researchers find ways to better serve families with fragile X.
Fragile X syndrome cascade testing and genetic counseling protocols
The 2005 National Society of Genetic Counselors guidelines for fragile X-related disorders were updated to include recent research on the different conditions that can affect people who have changes in the FMR1 gene. In addition, efforts were taken to help inform health care providers about these conditions.
Development of a newborn screening test for fragile X syndrome
CDC has worked with researchers to develop a test to detect fragile X syndrome in newborns. This project will also find out how often fragile X syndrome occurs in newborns.
Single Gene Resource Center
CDC is working with the Genetic Alliance to help people find quality information on single gene disorders, such as fragile X syndrome.