Fragile X Syndrome
(Martin-Bell Syndrome; FXS)
by Rick Alan
Fragile X syndrome (FXS) is a problem of the X chromosome. It is a condition that is inherited from the parents. FXS is the most common cause of inherited intellectual disability.
FXS is caused by problems with the FMR1 gene. These problems prevent fragile X mental retardation protein (FMRP) from developing. The protein helps to make connections in the brain. Without the protein, certain connections in the brain cannot be made. This causes developmental problems and prevents children from fully developing some higher cognitive functions.
The main risk factor for FXS is having a parent with an FMR1 mutation. Most people who inherit a minor mutation, which is sometimes called a premutation, do not develop the symptoms and signs of FXS. A few premutated children may show signs that resemble autism. Others, primarily males, may develop a set of neurological symptoms called fragile X tremor ataxia syndrome in later adult life. Females may develop FMR1-related premature ovarian insufficiency.
Children of mothers with FMR1 premutations are at risk of inheriting a fully mutated FMR1 gene severe enough to cause symptomatic FXS. If a woman does not have symptoms and is a carrier of an FMR1 mutation or premutation, each child of hers has a 50% chance of inheriting that gene. Of the children that inherit that gene, boys are much more likely to develop symptoms than girls. The severity of the disorder may vary between different individuals in the same family.
In general, each generation tends to have worse mutations and a higher risk of FXS than the previous one.
The number and severity of symptoms varies widely. Symptoms tend to be less frequent and milder in females. Symptoms can include:
You will be asked about your child's symptoms and medical history. A physical exam will be done. FXS may be suspected if a child has:
The diagnosis can be confirmed by genetic testing with a DNA blood test.
There is no cure for FXS. Treatment is aimed at controlling symptoms. Treatment may include:
Medications used to treat symptoms of FXS include:
Specific therapies depend on the degree of developmental delays, intellectual disability, or behaviors. These may include:
Specific educational strategies depend on the degree of developmental delays and/or intellectual disability. Educational strategies include setting a stable educational environment for the patient that includes:
There are no known ways to prevent FXS once a person is born. If you have FXS or risk factors for FXS, you can talk to a genetic counselor when deciding to have children.
FRAXA Research Foundation
The National Fragile X Foundation
Fragile X Research Foundation of Canada
Sick Kids—The Hospital for Sick Children
Bardoni B, Davidovic L, Bensaid M, Khandjian EW. The fragile X syndrome: exploring its molecular basis and seeking a treatment. Expert Rev Mol Med. 2006;8(8):1-16.
Committee on Genetics health supervision for children with fragile X syndrome. Pediatrics. 2011;127;994.
Fragile X syndrome. EBSCO DynaMed Plus website. Available at: http://www.dynamed... . Updated June 26, 2015. Accessed May 6, 2016.
Fragile X syndrome overview. National Institute of Child Health and Human Development website. Available at:
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Updated October 29, 2013. Accessed May 6, 2016.
What is fragile X? FRAXA Research Foundation website. Available at:
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Accessed June 24, 2013.
Last reviewed May 2018 by Michael Woods, MD
Last Updated: 5/29/2014
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