Fragile X syndrome is a genetic abnormality on an X chromosome that leads to intellectual disability and behavior problems.

Fragile X syndrome is the most commonly diagnosed inherited cause of moderate intellectual disability, and boys are affected more often. Fragile X syndrome is second only to Down syndrome as a cause of intellectual disability in boys. However, unlike fragile X syndrome, Down syndrome is not inherited in most cases. For more information, see the National Fragile X Foundation.

The symptoms of fragile X syndrome are caused by an abnormality of a specific gene on the X chromosome. People have an excessive number of copies (often more than 200) of a small segment of deoxyribonucleic acid (DNA).

Children and adults with the syndrome may have physical, intellectual, and behavior problems. Features, which are often subtle, include delayed development; large, protruding ears; a prominent chin and forehead; and, in boys, large testes (most apparent after puberty). The joints may be abnormally flexible, and heart disease (mitral valve prolapse) may occur. Children may have mild to moderate intellectual disability. Features of autism may develop, including repetitious speech and behavior, poor eye contact, and social anxiety. Women may experience menopause in their mid 30s.

Fragile X syndrome can be detected by DNA tests done after birth, typically when the child is school age or an adolescent. Boys with autism and intellectual disability should be tested for fragile X syndrome.

Early intervention, including speech and language therapy and occupational therapy, can help children with fragile X syndrome maximize their abilities. Stimulants, antidepressants, and antianxiety drugs may be beneficial for some children.

Last full review/revision April 2013 by Nina N. Powell-Hamilton, MD