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Fragile X


Fragile X syndrome (or Martin-Bell syndrome) is a genetic condition involving changes in part of the X chromosome. It is the most common form of inherited intellectual disability in boys and is also associated with a range of health complications. 

Causes of Fragile X

Fragile X is caused by a change in a gene called FMR1. A small part of the gene code is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem. Boys and girls can both be affected, but because boys have only one X chromosome, boys are usually affected more severely. Children can still have fragile X even if their parents do not have it. 


Diagnosis is made by a blood test. This genetic testing is usually suggested when symptoms or seen. For example, when a child has a large head or has delays in cognitive development. It is recommended that all children with unexplained intellectual disability have testing for fragile X.

How Many People Have Fragile X?

About 1 of every 4,000 males and 1 of every 8,000 females has fragile X syndrome. 

Associated Developmental and Learning Issues

Fragile X syndrome is the most common genetic (inherited) condition that causes intellectual and developmental delays. The severity of these delays depends on how much protein was actually reduced by the FMR1 gene.

  • Movement/mobility difficulties: Most children with fragile X have trouble with coordination and strength. Young children may be slow in developing motor skills.
  • Communication difficulties: Many children with fragile X develop speech skills later than other children their age.
  • Learning: Children with fragile X have a range of intellectual abilities. Some children have normal intellectual abilities. Others have problems ranging from mild learning disabilities to severe intellectual disability.
  • Social Skills: Many children with fragile X have difficulty learning to interact with other people and develop age appropriate friendships.

Associated Medical Conditions

Many children with fragile X have other health complications. It is important that these are recognized and treated. A child with fragile X who is physically healthy is more likely to do well in school and in other treatments. This contributes to more independence, better daily functioning, and better quality of life. 

Because of this, it is important for a child’s primary care provider to know about fragile X and how to look for common problems. Guidance for primary care providers is available through the American Academy of Pediatrics, titled Health Supervision for Children with Fragile X Syndrome.

Common Health Issues

Many of the physical problems associated with fragile X are related to loose connective tissue.

  • Low muscle tone (hypotonia)
  • Flat feet
  • Frequent infections in children, especially ear infections
  • Mitral valve prolapsed (heart problem)
  • Seizure disorder
  • Early puberty
  • Eye and vision problems
  • Urinary reflux
  • Inguinal hernia
  • Scoliosis
  • GI problems (GERD, constipation)

Fragile X itself does not get worse over time. However, how it affects a person may be different at different points in the person’s life. Behavioral symptoms can get better, worse, or stay the same during the course of a person’s life. Certain complications, such as tremor may develop in adulthood. Although fragile X is not "curable" in the usual sense, treatment can help improve function and prevent complications.

Associated Behavioral Conditions

  • Autism spectrum disorder
  • Intellectual disabilities
  • ADHD
  • Anxiety
  • Feeding problems
  • Self-injurious behavior

Related Services

URMC Collaborations


You can find resources for fragile X in our Resource Directory!