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Charity registration number 1127861 - The Fragile X Society Registered Charity and Limited Company Registered in England

Charity Registration SC047332 - The Fragile X Society Registered Charity in Scotland
Company registration number 6724061 - Registered office: Rood End House, 6 Stortford Road, Great Dunmow, Essex CM6 1DA


Fragile X Carriers

Fragile X Premutation

Large alterations to the FMR1 gene (over 200 CGG repeats) cause Fragile X Syndrome.  However, some people have small alterations to this gene (55-200 repeats).  People with this version of the gene are described as being carriers, or having a premutation. Approximately 1 in 250 females and 1 in 800 males are carriers.


There is a growing realisation that, although the small alteration is not enough to "switch off" the gene, like in Fragile X Syndrome, these smaller changes are important.


  • Firstly, the small alteration to the FMR1 gene makes it 'unstable'. This means that it might change size when it is passed to the next generation. Therefore, this can lead to future generations having Fragile X Syndrome. All known cases of Fragile X Syndrome have been caused when the carrier gene has expanded when being passed from a mother to her child. Fathers pass X chromosomes (where the FMR1 gene is located) to their daughters, however there are no known cases where the gene has expanded to cause Fragile X Syndrome when passed from father to daughter. You can find out more about the inheritance of Fragile X in our booklet.


  • In addition, some carriers will experience one of the medical conditions which are associated with the Fragile X premutation.  Fragile X Tremor Associated Ataxia (FXTAS) is a neurological condition which onsets later in life and some women may experience premature ovarian failure (FXPOI).  You can find out more about these conditions by clicking the buttons below. 

  • Finally, There is some evidence that individuals with the premutation may be more predisposed to experiencing particular mental health and cognitive characteristics or challenges. To find out more, contact our Families and Professionals Advisors who can send information and discuss any questions which you may have.