Fragile X Syndrome Fragile X Syndrome
Centers for Disease Control and Prevention

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.