Fragile X Syndrome

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.

To continue reading this article, you must login.
  • Research health conditions
  • Check your symptoms
  • Prepare for a doctor's visit or test
  • Find the best treatments and procedures for you
  • Explore options for better nutrition and exercise
Learn more about the many benefits and features of joining Harvard Health Online »