Fragile X Syndrome

Fragile X syndrome is caused by a mutation in the FMR1 (fragile X mental retardation 1) gene on the X chromosome. The FMR1 gene codes for the fragile X mental retardation protein, which plays a role in cognitive development in the brain.

It is X-linked, but it is unclear whether it is dominant or recessive. Males are always affected, but females can vary in how much they are affected. This is because females have a spare normal copy of the FMR1 gene on their other X chromosome. When the mother is phenotypically normal, the affected child may have inherited the X chromosome from their mother, or it may result from a de novo (random) mutation.



Fragile X syndrome usually presents with a delay in speech and language development. Other features are:

  • Intellectual disability
  • Long, narrow face
  • Large ears
  • Large testicles after puberty
  • Hypermobile joints (particularly in the hands)
  • Attention deficit hyperactivity disorder (ADHD)
  • Autism
  • Seizures



There is no cure for the condition. Management is supportive and involves treating the symptoms. This involves the multidisciplinary team to support the learning disability, manage autism and ADHD and treat seizures if they occur. Life expectancy is similar to the general population depending on associated disabilities and complications.


Last updated January 2020