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Fragile X Syndrome (FXS)

Fragile X Syndrome (FXS) is a genetic disorder caused by a mutation (a change in the DNA structure) in the X chromosome. It results in a wide range of developmental, physical and behavioral problems and is the most common known cause of inherited developmental disability worldwide.

1 in 2,500
males

1 in 5,000
females

One in 2,500 males and one in 5,000 females are affected. One in 250 females carries the premutation and will pass the gene on to 50% of her offspring, male or female. Boys and girls can both be affected, but because boys have only one X chromosome, a single fragile X is likely to affect them more severely.

You can have fragile X syndrome even if your parents do not have it; a family history of fragile X syndrome, developmental problems, or intellectual disability may not be present.

The genetic disorder is a mutation in a gene on the ‘X’ chromosome called FMR1 (fragile X mental retardation gene 1). This gene normally contains a repeat code of 6 – 50 ‘CGG’ triplet repeats. Premutation carriers have a small change in the FMR1 gene (a repeat code of 50 –200), whereas the full mutation tends to be affected individuals with a large change in the gene (more than 200 repeats). The mutation turns the gene off, thus causing the problems associated with the syndrome.

Features usually include:

  • intellectual disabilities, ranging from mild learning disabilities to more severe intellectual impact
  • attention deficit and hyperactivity, particularly in young children
  • anxiety and unstable mood
  • autistic behaviors and is the most common known genetic (“single gene”) cause of autism
  • sensory integration problems, such as hypersensitivity to loud noises or bright lights
  • speech delay, with expressive language more severely affected than receptive language.
  • seizures (epilepsy) affect about 25% of people with Fragile X

FXS is part of a family of genetic conditions known as Fragile X-associated disorders (FXD). Other conditions include Fragile X-associated tremor/ataxia syndrome (FXTAS), Fragile X-associated primary ovarian insufficiency (FXPOI), and Fragile X-Associated Neuropsychiatric Disorders (FXAND).

Learn more about FXS and related disorders here.

Resources about Fragile X Syndrome