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Fragile X Syndrome and Autism

Interactive Autism Network at Kennedy Krieger Institute
Date Last Revised: 
March 1, 2018
Date Published: 
April 2, 2007

Photo of chalk drawing, genetics and double helixAutism occurs more frequently in certain disorders, including fragile X syndrome, or FXS. Fragile X, the most common cause of inherited intellectual disability, is caused by the mutation of a single gene, called FMR1, on the X chromosome.

The gene, which is supposed to make a protein needed for typical brain development, is turned partially or totally off.1 Both boys and girls are affected, but because boys – who have one “X” and one “Y” chromosome – have no back-up for a problem on the “X” chromosome, they are more severely affected than girls, who have two “X” chromosomes. Boys usually have some degree of intellectual disability. Girls with the condition may have intellectual disability or normal intelligence.2

An estimated 1 in 5,000 boys are born with FXS.2 People with FXS often have developmental delays, and a distinct physical appearance, including a large head, long face, and prominent ears.3 A national survey of parents of children with FXS found that 46 percent of the boys and 16 percent of the girls had been diagnosed or treated for autism.2, 4 Those with both FXS and autism face more challenges than those with FXS alone, and early intervention is especially crucial for them.5

Some studies found that as many as 60% of males with Fragile X Syndrome could be diagnosed with ASD based on their significant and frequent autistic behaviors.6-8

It’s unclear how many people with autism also have fragile X. About 10% of children with autism also have Down syndrome, fragile X syndrome, tuberous sclerosis, or other genetic disorders.9

Screening for fragile X and autism

Understanding that fragile X and autism often occur together is important both clinically and scientifically. In clinical terms, the connection means that those with autism can be screened for fragile X with a blood test. Also, children with fragile X can be carefully watched for signs of autism. Finding out fragile X is playing a role is important to families because the condition is genetic, and can affect other members of the family. For example, the sibling of a child with fragile X, who has been having more mild difficulties, may also be affected by fragile X, though to a different degree. Other relatives, who may be carriers of the condition even if they do not have it themselves, will be able to benefit from genetic counseling when it comes time to make decisions about having children.

In terms of science, it is hoped that the overlap between fragile X and autism may point the way to answers. A diagnosis of autism is currently made based entirely on behavior. There is no known cause, and no known biological marker of the condition. (In other words, you can’t draw a blood sample, look at an MRI, or do a genetic test to tell if a person has autism.) In contrast, a diagnosis of fragile X is made entirely based upon genetics, and the multiple influences of this one mutation on structure and processing in the brain are at least partially understood. It is hoped that, by using fragile X as a starting point, progress may be made in solving the mysteries surrounding autism.10

In recent years, some researchers have been looking at whether autism in boys with fragile X syndrome differs from autism in boys who don’t have a genetic syndrome. That information could shed light on the development and treatment of both conditions. One study found that the boys with FXS has significantly less trouble with social smiling, but more trouble with complex mannerisms, than similar boys who do not have FXS.11

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  1. U.S. Centers for Disease Control and Prevention. (2006). Causes of Fragile X Syndrome. Retrieved on November 22, 2010.
  2. U.S Centers for Disease Control and Prevention. (2015) Facts about Fragile X Syndrome. Retrieved on July 6, 2015 from
  3. U.S. Centers for Disease Control and Prevention. (2006). Fragile X Syndrome. Retrieved on November 22, 2010.
  4. Bailey, D. B., Raspa, M., Olmsted, M. and Holiday, D. B. (2008), Co-occurring conditions associated with FMR1 gene variations: Findings from a national parent survey. American Journal of Medical Genetics, 146A: 2060–2069. Article.
  5. Bailey, D.B., Hatton, D.D., Skinner, M., & Mesibov, G. (2001). Autistic behavior, FMR1 protein, and developmental trajectories in young males with Fragile X syndrome. Journal of Autism and Developmental Disorders, 31(2), 165-174.  Abstract
  6. Clifford, S., Dissanayake, C., Bui, Q. M., Huggins, R., Taylor, A. K., & Loesch, D. Z. (2007). Autism spectrum phenotype in males and females with fragile X full mutation and premutation. Journal of Autism and Developmental Disorders, 37(4), 738–747.
  7. Harris, S. W., Hessl, D., Goodlin-Jones, B., Ferranti, J., Bacalman, S., Barbato, I., et al. (2008). Autism profiles of males with fragile X syndrome. American Journal on Mental Retardation, 113(6), 427–438.
  8. McDuffie, A., Abbeduto, L., Lewis, P., Kover, S., Kim, J., Weber, A., et al. (2010). Autism spectrum disorder in children and adolescents with fragile X syndrome: Within-syndrome differences and age-related changes. American Journal on Intellectual and Developmental Disabilities, 115(4), 307–326.
  9. U.S. Centers for Disease Control and Prevention. (2015) Autism Spectrum Disorder Data and Statistics. Retrieved on July 6, 2015, from
  10. Belmonte, M.K., & Bourgeron, T. (2006). Fragile X syndrome and autism at the intersection of genetic and neural networks. Nature Neuroscience, 9(10), 1221-1225. Abstract
  11. McDuffie, A., Thurman, A.J., Hagerman, R.J. & Abbeduto, L. (2015) Symptoms of Autism in Males with Fragile X Syndrome: A Comparison to Nonsyndromic ASD Using Current ADI-R Scores. Journal of Autism and Developmental Disorders 45(7):1925-37. Abstract.