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Fragile X Syndrome

Updated: December 21, 2018

What Is It?

Fragile X syndrome is an inherited disorder associated with mental retardation and a particular appearance. It is caused by an error in a small piece of the DNA (genetic blueprint) for the FMR1 gene. This gene is found on the X chromosome, one of the two chromosomes (X and Y) that determine gender. 

In people with fragile X, a particular section of the DNA code — CGG — is "fragile" and gets repeated an unusually large number of times, compared with the normal 5 to 50 times. As a result, the FMR1 gene is not expressed (turned on), and the body cannot produce FMR1 protein, which is related to nerve function. 

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