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Fragile X syndrome is a genetic condition involving changes in part of the X chromosome . It is the most common form of inherited mental retardation in males and a significant cause of mental retardation in females.

Alternative Names:

Martin-Bell syndrome; Marker X syndrome

Causes, incidence, and risk factors:

Fragile X syndrome is caused by a change in the FMR1 gene. A small section of the gene code (three letters only -- CGG) is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem.

Normally, the FMR1 gene makes a protein needed for your brain to grow properly. A defect in this gene makes your body produce too little of the protein, or none at all.

Boys and girls can both be affected, but because boys have only one X chromosome, a single fragile X is likely to affect them more severely. You can have Fragile X syndrome even if your parents do not have it.

  • Hyperactive behavior
  • Large body size
  • Large forehead or ears with a prominent jaw
  • Large testicles (macro-orchidism) after the beginning of puberty
  • Mental retardation
  • Tendency to avoid eye contact

Family members who have fewer repeats in the FMR1 gene may not have mental retardation, but may have other problems. Women with less severe changes may have premature menopause or difficulty becoming pregnant. Both men and women may have problems with tremors and poor coordination.

Signs and tests:

The person will have a family history of Fragile X syndrome (especially a male relative).

There are very few outward signs of Fragile X syndrome in babies. Some signs may include:

  • Large head circumference in babies
  • Mental retardation
  • Oversized testes in males who have reached puberty
  • Subtle differences in facial characteristics

In females, excess shyness may be the only sign of the problem.

A genetic test called polymerase chain reaction (PCR) is used to diagnose this disease. This test looks for a mutation (called a triplet repeat) in the FMR1 gene.

In the past, a specific type of chromosome analysis was done. Such testing may still be available.


There is no specific treatment for Fragile X syndrome. Instead, training and education help affected children function at as high a level as possible. Because the condition is not rare, educational approaches have been developed and tested.

Support Groups:

National Fragile X Foundation -- www.nfxf.org

Expectations (prognosis):

The outcome depends on the extent of mental retardation.


Complications vary depending on the type and severity of symptoms.

Calling your health care provider:

Call your health care provider if you suspect Fragile X syndrome in a child with mental retardation. Genetic counseling is recommended to help families understand the condition, and to understand the complex nature of DNA testing in Fragile X.


Genetic counseling may help both existing and prospective parents with a family history of Fragile X syndrome, or a family history of other symptoms such as tremor. Genetic testing can help determine the level of risk in these families.

Accurate diagnosis is important, because other family members can inherit Fragile X syndrome or other problems related to an increased number of repeats in FMR1.

Review Date: 8/11/2009
Reviewed By: Diana Chambers, MS, EdD, Certified Genetics Counselor (ABMG), Charter Member of the ABGC, University of Tennessee, Memphis. Review provided by VeriMed HealthcareNetwork. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

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Wentworth-Douglass Hospital
789 Central Avenue, Dover, NH 03820
Phone: (603) 742-5252
Toll free: 1 (877) 201-7100