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What Is Fragile X Syndrome?

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Fragile X syndrome (also called Fragile X) is the most common inherited form of mental retardation.* It results from a change, or mutation, in a single gene, which can be passed from one generation to the next. Fragile X appears in families of every ethnic group and income level.

Symptoms of Fragile X syndrome occur because the mutated gene cannot produce enough of a protein that is needed by the body’s cells, especially cells in the brain, to develop and function normally. The amount and usability of this protein, in part, determine how severe the effects of Fragile X are.

The most noticeable and consistent effect of Fragile X is on intelligence. More than 80 percent of males with Fragile X have an IQ (intelligence quotient) of 75 or less. 1 The effect of Fragile X on intelligence is more variable in females. Some females have mental impairment, some have learning disabilities, and some have a normal IQ.

People with Fragile X syndrome also share certain medical problems as well as many common physical characteristics, 2 such as large ears and a long face. In addition, having Fragile X is often associated with problems with sensation, emotion, and behavior.

The National Institute of Child Health and Human Development (NICHD), part of the National Institutes of Health (NIH), is a major sponsor of research on Fragile X syndrome. Since 1991, when researchers funded by the NICHD discovered the gene that causes Fragile X, scientists have learned a great deal about that gene’s structure and functions. The NICHD continues to support clinicians and scientists around the world who are working to find effective behavioral or other therapies, medical treatments, and prevention strategies for Fragile X.

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*The NICHD recognizes that there is a debate about the use of the term “mental retardation,” and that self-advocacy groups and professional associations are currently discussing alternative terms. Until a consensus is reached, and with the goal of addressing health-related issues faced by people with what has been traditionally known as “mental retardation,” this booklet uses that term to describe some features related to Fragile X syndrome. For more information, please see the Additional Resources section of this booklet

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Last Updated Date: 08/18/2006
Last Reviewed Date: 08/18/2006
Vision National Institutes of Health Home BOND National Institues of Health Home Home Storz Lab: Section on Environmental Gene Regulation Home Machner Lab: Unit on Microbial Pathogenesis Home Division of Intramural Population Health Research Home Bonifacino Lab: Section on Intracellular Protein Trafficking Home Lilly Lab: Section on Gamete Development Home Lippincott-Schwartz Lab: Section on Organelle Biology