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Similarities and Differences in Fragile X Syndrome and Autism

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Most people have heard of autism, but many are not familiar with an associated disorder: Fragile X syndrome (FXS). Fragile X syndrome is the leading genetic cause of autism and the most common cause of inherited intellectual disability. According to the Centers for Disease Control and Prevention, Fragile X and other genetic or chromosomal conditions are responsible for about 10 percent of autism cases.1 On the other hand, nearly half of all males with FXS have also been diagnosed with autism.2

While there is overlap between the two conditions, there are also key differences. In honor of July as Fragile X Awareness Month, we’re shedding some light on the causes, symptoms and treatment of Fragile X syndrome and its link to autism spectrum disorders.

Cause: Fragile X is caused by a genetic mutation in the FMR1 gene on the X chromosome. The causes of autism are less clear.

Like other X-linked conditions, Fragile X syndrome is caused by a genetic mutation on the X chromosome and typically is passed down from carrier mothers to male children. Females have two X chromosomes, which means that females with a mutated FMR1 gene usually also have a working copy of the gene. Because this working copy compensates for the changed copy, females may be carriers of Fragile X but typically are not affected by it. When females are affected, they are more likely to exhibit milder symptoms than males with Fragile X. Males only have one X chromosome and therefore a male with a mutation in the FMR1 gene will have the disorder.

Unlike with FXS, there’s no one cause of autism spectrum disorders. Research shows that a combination of genetic changes and environmental factors may increase a person’s risk of autism.3 While the genetics of autism are not yet well understood, there’s evidence to suggest that autism may run in families.3

Symptoms: Individuals with Fragile X often have intellectual disabilities. Individuals with autism have difficulties with social interactions and communication.

Although symptoms of Fragile X and autism spectrum disorders overlap, FXS is characterized by intellectual disability and developmental delays while autism is defined by challenges with social interaction and language impairment. Since some individuals have both disorders, the line between signs of FXS and autism can be blurry. Even those who don’t have both disorders may exhibit similar symptoms, such as repetitive behaviors and poor eye contact.

Symptoms of both Fragile X and autism range in severity. Some individuals may experience relatively mild symptoms while others have more severe forms of the disorder(s).

Treatment: There currently is no cure for Fragile X syndrome or autism spectrum disorders. However, early intervention and treatment can help.

While there currently is no cure for Fragile X or autism, early intervention may help improve a child’s development.4 If your family is affected by one or both of these disorders, a team of doctors and health professionals can work with you to develop an individualized treatment plan that best meets your needs.  

In some cases, genetic testing may provide a diagnosis that can help guide treatment. Fragile X testing is recommended for those with a family history of the disorder and for individuals with autism. Fragile X testing is often performed in conjunction with a microarray (another genetic test) for children with autism, intellectual disability, or developmental delay of unknown cause.  Carrier screening for Fragile X is also available to all women preconception and included as part of the Center’s screening program unless otherwise requested.

Questions? Our genetic counselor is your resource in the community for information about Fragile X and genetic testing.


  1. https://www.cdc.gov/ncbddd/autism/data.html
  2. https://www.cdc.gov/ncbddd/fxs/data.html
  3. https://www.autismspeaks.org/what-autism/learn-more-autism/what-causes-autism
  4. https://www.cdc.gov/ncbddd/fxs/facts.html

Photo credit: Darryl Leja, NHGRI



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