What are the causes and symptoms of Fragile X?

By Alan Greene, MD, FAAP
October 03, 2005


What exactly is fragile X? Does it have any symptoms?


Fragile X is a syndrome where there is an expanded segment of DNA on the X-chromosome. Because boys have only one X chromosome, they are most affected by this problem. In fact, fragile X is the number one cause of mental retardation in boys. When a woman passes the fragile X premutation, the affected segment of DNA is expanded again. Normally there are up to 50 repeats. Two hundred to 2,000 or greater repeats clearly causes symptoms and in between, with 52 to 200 repeats, causes what is called premutation.

I'm not aware that any symptoms exist, but fragile X is an active area of research. It is not visible on traditional chromosome studies and is sometimes tough to see, even if the chromosomes are prepared specially to look for it, so now it is diagnosed and studied using much more precise DNA studies.

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