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Fragile X syndrome


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


Fragile X syndrome is caused by a change in the FMR1 gene. The gene's code 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 you need for 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.


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

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.

Exams 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. Babies may have a large head circumference. Measurement of oversized testes in males who have reached puberty may also suggest the diagnosis. An experienced geneticist may note subtle differences in facial characteristics. Mental retardation is the hallmark of this condition and, in females, this may be the only sign of the problem.

A specific genetic test called polymerase chain reaction (PCR) is used to diagnose this disease. This test looks for an expanded 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, effort is directed toward training and education so that affected children can function at as high a level as is possible. Because the condition is not rare, specific educational approaches have been developed and tested.

Support Groups

National Fragile X Foundation --

Outlook (Prognosis)

The outcome depends on the extent of mental retardation.

Possible Complications

Complications vary depending on the type and severity of symptoms.

When to Contact a Medical Professional

Call your health care provider if you suspect the possibility of Fragile X syndrome in a child with mental retardation.


Genetic counseling may help prospective parents with a family history of Fragile X syndrome. Genetic testing can help determine the level of risk in such a family. Accurate diagnosis is important because of the possibility of other family members inheriting either Fragile X syndrome or other problems related to an increased number of repeats in FMR1.

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