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Fragile X syndrome

What is Fragile X syndrome (FXS)?
Fragile X syndrome (also called FXS) is the most common cause of inherited mental retardation.  It is also the most common known cause of autism.  It affects about 1 in 4000 males and 1 in 8000 females [1]. It can cause a range of mental impairment—varying from mild to severe, and can also cause many other typical features and symptoms.  It can affect all racial and ethnic groups.

To learn more about genetics and to better understand how genes cause syndromes, see Your Child: Genetic Syndromes.

What causes Fragile X syndrome?

View a video about what causes FXS, from the Centers for Disease Control and Prevention (CDC).

What are the features of Fragile X Syndrome?
These features may or may not be present in affected people.  They can range greatly in how severe they are.

More about physical features of males and females with FXS, as well as a screening checklist for infants and toddlers, from the Fragile X Information Center at the University of North Carolina.

More about behavior from the Fragile X Information Center.

Who should have the FXS test?
Genetic testing is available to diagnose FXS and to test for female carriers. The DNA test provides definite diagnosis and very accurate carrier detection.  Testing is recommended for:

Prenatal testing is possible for fetuses (babies). Pregnant mothers who are known carriers should talk to their doctors about having their fetus tested.

For more information on testing to diagnose FXS and for carrier status, see this practice guideline of the American College of Medical Genetics.

What is the treatment for FXS?
There is no cure for FXS, but there are many treatments to help your child.

What do I need to know about the health of my child with FXS?

What are some more resources for information and support?

[1] Fragile X syndrome.  Genetics Home Reference.  Reviewed May 2007.  Accessed 22 May 2007.  Available from:

[2] Subramony SH, Friedrich CA, Jankowiak J. Fragile X and company: finding the right diagnosis. Neurology. 2005 Jul 26;65(2):E3-4.

Written and compiled by by Kyla Boyse, R.N., Tarin Gitlin, M.D. and Elizabeth Rzepka, M.D.  
Reviewed by faculty and staff at the University of Michigan.
Updated March 2010

U-M Health System Related Sites:
U-M Pediatrics
C.S. Mott Children's Hospital

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