Skip to Content

Fragile X Syndrome


Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Last full review/revision Jul 2020| Content last modified Jul 2020

Fragile X syndrome is a genetic abnormality on the 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 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 55 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.

Symptoms of Fragile X Syndrome

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 the premutation may go through menopause at a much younger age, especially in their mid-30s.

People who have the premutation are at risk of Fragile X–associated tremor/ataxia syndrome (FXTAS), which causes tremor, loss of coordination, and deterioration of mental function.

Diagnosis of Fragile X Syndrome

  • 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. Boys with autism and intellectual disability should be tested for Fragile X syndrome, especially if their mother has relatives who have similar problems.

Treatment of 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.

More Information about Fragile X Syndrome

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

  • National Fragile X Foundation: Provides advocacy, education, support, and public and professional awareness programs and services

Copyright © 2022 Merck & Co., Inc., known as MSD outside of the US, Kenilworth, New Jersey, USA. All rights reserved. Merck Manual Disclaimer