What is Fragile X Syndrome (FXS)

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Fragile X syndrome (FXS) is a genetic condition that causes intellectual disability, behavioral and learning challenges, and various physical characteristics. Though FXS occurs in both genders, males are more frequently affected than females, and generally with greater severity. Life expectancy is not affected in people with FXS because there are usually no life-threatening health concerns associated with the condition.



FXS has been detected in all populations and ethnic groups. As a result, efforts have been made to determine the overall prevalence of FXS and the difference in prevalence between males and females.

There have been several studies undertaken both in the “special needs” population and the general population aimed at determining the prevalence of FXS in males and females. The agreed upon prevalence of FXS in males is approximately 1 in 7,000 and in females 1 in 11,000.

The reason there are fewer females with FXS than males is that the gene for FXS is located on the X chromosome.

  • Males (XY) — having only one X chromosome — will develop FXS because they have a mutation of their single (only) X chromosome.
  • Females (XX) — having two X chromosomes — can have the unaffected X reduce the effects of the affected X, which typically leads to no or milder symptoms of FXS. (This is an important distinction, as many females with the full mutation do not consider themselves, nor are they considered by others, to “have” FXS.)

While researchers do not have an exact number for how many Americans (males and females) could have full-mutation Fragile X syndrome, the ratios noted above suggest that the raw number of individuals could be as low as 38,000 or as high as 87,000. (Worldwide, the number could be between 777,000 and 1,400,000.) However, it’s important to note that some published papers suggest greater prevalence and some lower prevalence than the numbers cited above.


Signs & Symptoms

In Males:

Behavioral characteristics can include ADD, ADHD, autism and autistic behaviors, social anxiety, hand-biting and/or flapping, poor eye contact, sensory disorders, and increased risk for aggression.

Intellectual disabilities in FXS include a range from moderate learning disabilities to more severe intellectual disabilities. The majority of males with Fragile X syndrome demonstrate significant intellectual disability.

Physical features may include large ears, long face, soft skin, and large testicles (called “macroorchidism”) in post-pubertal males. Connective tissue problems may include ear infections, flat feet, high arched palate, double-jointed fingers, and hyper-flexible joints. No one individual will have all the features of FXS, and some features, such as a long face and macroorchidism, are more common after puberty.

Disposition: They are also very social and friendly, have excellent imitation skills, have a strong visual memory/long term memory, like to help others, are nice, thoughtful people, and have a wonderful sense of humor.


In Females:

Behavioral characteristics seen in males can also be seen in females, though females often have milder intellectual disability and a milder presentation of the syndrome’s behavioral and physical features.

Intellectual disabilities: About one-third of females with FXS have a significant intellectual disability. Others may have moderate or mild learning disabilities, emotional/mental health issues, general anxiety, and/or social anxiety.

A small percentage of females who have the full mutation of the FMR1 gene that causes FXS will have no apparent signs of the condition—intellectual, behavioral, or physical. These females are often identified only after another family member has been diagnosed.

*The information in this blog was taken from fragilex.org. Fragilex.org’s mission is to serve the entire Fragile X community to live their best lives by providing the knowledge, resources, and tools until, and even after, more effective treatments are achieved.



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