Fragile X syndrome

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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 intellectual disability in boys.

Alternative Names

Martin-Bell syndrome; Marker X syndrome


Fragile X syndrome is caused by a change in a gene called FMR1. A small part of the gene code is repeated on a fragile area of the X chromosome. The more repeats, the more likely the condition will occur.

The FMR1 gene makes a protein needed for your brain to function properly. A defect in the 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.

A family history of fragile X syndrome, developmental problems, or intellectual disability may not be present.


Behavior problems associated with fragile X syndrome include:

Physical signs may include:

  • Flat feet
  • Flexible joints and low muscle tone
  • Large body size
  • Large forehead or ears with a prominent jaw
  • Long face
  • Soft skin

Some of these problems are present at birth, while others may not develop until after puberty.

Family members who have fewer repeats in the FMR1 gene may not have intellectual disability. Women may have premature menopause or difficulty becoming pregnant. Both men and women may have problems with tremors and poor coordination.

Exams and Tests

There are very few outward signs of fragile X syndrome in babies. Some things the health care provider may look for include:

  • Large head circumference in babies
  • Intellectual disability
  • Large testicles after the start of puberty
  • Subtle differences in face features

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

Genetic testing can diagnose this disease.


There is no specific treatment for fragile X syndrome. Instead, training and education have been developed to help affected children function at the highest possible level. Clinical trials are ongoing ( and looking at several possible medicines for treating fragile X syndrome.

Support Groups

National Fragile X Foundation:

Outlook (Prognosis)

How well the person does depends on the amount of intellectual disability.

Possible Complications

Complications vary, depending on the type and severity of symptoms. They may include:

  • Recurrent infections in children
  • Seizure disorder

Fragile X syndrome can be a cause of autism or related disorders, although not all children with fragile X syndrome have these conditions.


Genetic counseling may be helpful if you have a family history of this syndrome and are planning to become pregnant.


Saul RA, Tarleton JC. FMR1-related disorders. GeneReviews. Seattle, WA: University of Washington; 2012:4. PMID: 20301558 Accessed August 1, 2015.

Shapiro BK, Batshaw ML. Intellectual disability. In: Kliegman RM, Stanton BF, St Geme JW III, Schor NF, eds. Nelson Textbook of Pediatrics. 20th ed. Philadelphia, PA: Elsevier; 2016:chap 36.

Version Info

  • Last reviewed on 8/1/2015
  • Chad Haldeman-Englert, MD, FACMG, Fullerton Genetics Center, Asheville, NC. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.

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