Fragile X

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Picture courtesy of FRAXA Research Foundation

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For healthcare providers: Additional clinical information

Fragile X syndrome is a condition characterized by intellectual disability and physical features, that is more common in boys than in girls although it affects both. It is caused by a change in a gene (FMR1) on the X chromosome. The condition is characterized by mild to moderate intellectual disability in males and normal to mild intellectual disability in females. There is variability in the level of intellectual disability and other behavioral and physical features.

Fragile X is the result of one of two types of gene mutations. It can either be a full mutation (FMR1) or a loss of function mutation. Females have some of the characteristics and physical features as males who have Fragile X syndrome. There is variability of characteristics associated with this condition.

About 1 in every 3,600-4,000 baby boys is born with Fragile X, and 1 in 4,000-6,000 baby girls.

Learn more about the physical characteristics and/or symptoms of Fragile X

The diagnosis is made based on genetic testing. The testing identifies a difference in the FMR1 gene. The FMR1 gene produces a protein important for normal brain development. Fragile X syndrome is caused by an increase in the number of repeats (CGG repeats) of a portion of the FMR1 gene that causes the gene to not be able to make the protein.

The increase in the number of repeats can be small and results in the person being a carrier, or having a 'pre-mutation'. Fragile X carriers or FMR1 pre-mutation carriers may not experience any symptoms.  A pre-mutation carrier may “carry” the potential to pass the pre-mutation on to their offspring. Learn more about the genetics of Fragile X syndrome.

MALES - The clinical findings in males with a full mutation may include the following:

  • Specific facial appearance and physical findings. Not every individual will have all of the findings.
  • Intellectual disability is present. Males with full mutation have a significant intellectual disability (IQ may range from 22-65).
  • Behavioral differences may occur. Other diagnoses/characteristics may include:
    • Autism spectrum disorder
    • Hyperactivity
    • Shyness
    • Gaze aversion
    • Hand flapping
    • Hand biting
    • Temper tantrums  
  • Other findings may include: low muscle tone, reflux, repeated otitis media, high blood pressure, possible seizures.

FEMALES - Clinical finding in females who have a full mutation:

  • Females have some of the same physical features as males.
  • Intellectual disability may be found in up to 50% of females. Females with a full mutation have a mild intellectual disability (IQ may range from 74-91). Learn More.

This information was last updated in May 2022.