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Fragile X Syndrome -ˈeks-

By Nina N. Powell-Hamilton, MD, Clinical Assistant Professor of Pediatrics ; Medical Geneticist , Sidney Kimmel Medical College at Thomas Jefferson University; Nemours/Alfred I. duPont Hospital for Children

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

Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body (see Genes and Chromosomes for a discussion about genetics). Genes contain instructions that determine how the body is supposed to function.

Fragile X syndrome is the most commonly diagnosed inherited cause of moderate intellectual disability, and boys are affected more often than girls. 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 (see Gene abnormalities). People have an excessive number of copies (more than 200) of a small segment of deoxyribonucleic acid (DNA). People with 60 to 200 extra copies are considered to have a premutation because, although they do not have the disorder, their offspring are at greater risk of having it.


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 who have a premutation may go through menopause in their mid 30s.


  • DNA tests

Fragile X syndrome can be detected by DNA tests done after birth. The diagnosis of Fragile X syndrome is typically made when the child is school age or an adolescent. Children with autism and intellectual disability should be tested for Fragile X syndrome.


  • Speech, language, and occupational therapies

  • Sometimes drugs

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.

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