Prevalence

How Common is Fragile X Syndrome?

There have been a number of studies aimed to determine the prevalence of FXS in males and females. Studies have been undertaken both in the “special needs” population and the general population. The agreed upon prevalence of FXS in males is approximately 1 in 3600 to 4000 and in females is approximately 1 in 4000 to 6000. The reason it is lower in females is that, while all males with an FMR1 full mutation will have fragile X syndrome, some females with an FMR1 full mutation will not have behavioral, cognitive or physical features of FXS.

How Many Individuals are Carriers of the FMR1 Premutation?

General population studies in both the newborn and adult populations have found that approximately 1/250 females and 1/800 males are carriers of the FMR1 premutation worldwide. There may be some variation in some populations. For example a large Israeli study found approximately 1/130 women were FMR1 carriers. However, at this time, the accepted carrier rates internationally stand at 1/250 females and 1/800 males

The Prevalence of FXTAS and FXPOI

• Studies report that approximately 1/3 (33%) of all men over 50 years of age with an FMR1 premutation will develop symptoms of FXTAS. Not all of these men will have symptoms that fulfill the diagnostic criteria for FXTAS, as some develop mild features that do not progress.
• Studies report approximately 5-8% of women over 50 years of age with an FMR1 premutation, will develop features of FXTAS, though females tend to have fewer and milder symptoms than men.
• Approximately 20-25% of women with an FMR1 premutation will develop FXPOI. FXPOI covers a range of ovarian difficulties including early menopause, irregular menstrual cycles, infertility, sub fertility and premature ovarian failure (cessation of menstrual periods prior to age 40)

How Common are Intermediate (Grey Area) Alleles?

Approximately 1 in 50 (2%) of individuals have an intermediate allele. There appear to be no clinical associations with intermediate alleles. Most intermediate alleles are stable and do not change over generations. In a small number of families intermediate alleles show some slight instability and can lead to a premutation in future generations. Individuals with an intermediate allele are not at risk for any for the FXD’s or to have children with fragile X syndrome.

Based on the best available evidence:

• Approximately one-million Americans carry the Fragile X mutation, including approximately 100,000 with fragile X syndrome, and are at risk for developing a Fragile X-associated Disorder.
• Approximately 1 in 3600 to 4000 males in the world are born with the full mutation for Fragile X.
  Note: The vast majority of males with the full mutation will have fragile X syndrome.
• Approximately 1 in 4000 to 6000 females in the world are born with the full mutation for Fragile X.
  Note: Approximately 50% of females with the full mutation will have some features of fragile X syndrome.
• Approximately 1 in 800 men in the world are carriers of the Fragile X premutation.
• Approximately 1 in 260 women in the world are carriers of the Fragile X premutation.