Fragile X syndrome is caused by a CGG trinucleotide repeat expansion in the FMR1 (Fragile X Messenger Ribonucleoprotein 1) gene on the X chromosome. This encodes the fragile X messenger ribonucleoprotein, which plays an essential role in cognitive development.
Fragile X syndrome is inherited in an X-linked pattern. Males who inherit the affected X chromosome will inherit the condition. Females with one affected X chromosome and an unaffected X chromosome may also be affected, although the severity and features can vary.
Features
Fragile X syndrome typically presents with a delay in speech and language development. Other features include:
- Intellectual disability
- Autism spectrum disorder
- Long, narrow face
- Large ears
- Large testicles after puberty
- Joint hypermobility (particularly in the hands)
- Attention deficit hyperactivity disorder (ADHD)
- Seizures
Fragile X-associated tremor/ataxia syndrome (FXTAS) is also associated with the genetic abnormality. It presents in later adulthood with intention tremor, cerebellar ataxia and dementia.
FMR1 primary ovarian insufficiency (FXPOI) presents in females with ovarian failure before the age of 40 (early menopause).
Management
Management is supportive and involves treating the symptoms with input from the multidisciplinary team.
Life expectancy is similar to the general population, depending on associated disabilities and complications.
Last updated December 2025
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