What is Fragile X syndrome (FXS)?
Fragile X syndrome (also called FXS) is the most common cause of inherited mental retardation. It is also the most common known cause of autism. It affects about 1 in 4000 males and 1 in 8000 females . It can cause a range of mental impairment—varying from mild to severe, and can also cause many other typical features and symptoms. It can affect all racial and ethnic groups.
To learn more about genetics and to better understand how genes cause syndromes, see Your Child: Genetic Syndromes.
What causes Fragile X syndrome?
- It is inherited.
- It is caused by mutation of a gene (FMR1) on the X-chromosome. Normally, the gene makes a protein needed for brain development. But the mutation causes a person to make less or none of the protein, which causes FXS.
- People with a small change in the FMR1 gene might not show any sign of the syndrome. Bigger changes in the gene cause more severe symptoms.
- Genetic testing for FXS is possible.
- Mothers may be a carrier of the mutation and pass it on to their children.
- Or, a mother may have a pre-mutation that expands to a full mutation when passed on to the next generation.
- Boys are affected more severely than girls, because girls have another X chromosome, which usually does not have the mutation and can partially compensate for the nonfunctioning one. Boys have only one X chromosome (and one Y chromosome).
- The damaged gene can be passed along silently (without symptoms) for generations before a child is affected by FXS.
View a video about what causes FXS, from the Centers for Disease Control and Prevention (CDC).
What are the features of Fragile X Syndrome?
These features may or may not be present in affected people. They can range greatly in how severe they are.
- Prominent ears, large head, prominent forehead, flexible finger joints, high palate, flat feet.
- During puberty boys usually develop large testicles ( macroorchidism ) and a long face.
More about physical features of males and females with FXS, as well as a screening checklist for infants and toddlers, from the Fragile X Information Center at the University of North Carolina.
- Intelligence problems—ranging from mild learning disabilities to severe intellectual disability (Find out more about cognitive development at different ages and stages in FXS.)
- Developmental delays–especially speech and language (most often in boys)
- Attention Deficit Hyperactivity Disorder (ADHD) or poor attention span
- Autism and autistic behaviors—such as hand flapping, hand biting, chewing on clothes
- Find out more about autism and fragile X syndrome
- Social and emotional problems, such as aggression in boys or shyness in girls
- Very sensitive to stimuli
- Great sense of humor
- Excellent memory
- Frequent tantrums
More about behavior from the Fragile X Information Center.
Who should have the FXS test?
Genetic testing is available to diagnose FXS and to test for female carriers. The DNA test provides definite diagnosis and very accurate carrier detection. Testing is recommended for:
- Any person with developmental delay, signs of autism, a learning disability or intellectual disability (borderline to severe) of unknown cause
- Any person who has:
- any of the physical features or behaviors that are typical in FXS
- a relative who has FXS
- any relatives with intellectual disability of unknown cause
- Any person with autism, or who has autism-like behaviors
- Anyone who previously had the chromosome test (an older, costly, and often inaccurate test) for FXS
- Women with primary ovarian insuffiency (or POI, which used to be called primary ovarian failure, or POF), or “early menopause”, or with a family history of POI
- Couples who have one or more relatives with mental retardation of unknown cause may want to be tested before deciding to have a child.
- En Español: ¿Tengo Falla Ovarica Prematura?
Prenatal testing is possible for fetuses (babies). Pregnant mothers who are known carriers should talk to their doctors about having their fetus tested.
- About testing, if you find out someone in your family has Fragile X syndrome.
For more information on testing to diagnose FXS and for carrier status, see this practice guideline of the American College of Medical Genetics.
What is the treatment for FXS?
There is no cure for FXS, but there are many treatments to help your child.
- Early intervention in the pre-school years can help your child make better progress. See Your Child: Developmental Delay for more information about how to get started in an early intervention program. In Michigan the early intervention program is called Early On.
- To reach full potential, a child may need speech and language therapy, occupational therapy, and physical therapy to help with the many physical, behavioral, and cognitive impacts of FXS.
- Special education programs should fit your child's individual needs to modify classes and assignments. Children should be integrated into regular education whenever possible.
- Having a regular routine, avoiding over-stimulation, and using calming techniques can help reduce behavior problems.
- Medications may help treat the aggression, seizures, hyperactivity and short attention span that can come along with FXS.
- Treatment should be tailored to meet each child's specific needs.
- Usually children with FXS do not have many medical problems.
- Health problems may include: frequent ear infections, seizures, strabismus, mitral valve prolapse. Here is a summary of some of the health problems associated with FXS.
- This policy statement from the American Academy of Pediatrics on health supervision of children with Fragile X will help you and your pediatrician keep your child's health checks on track.
- Recently, it’s been discovered that carriers of the pre-mutation can have a neurological disorder called fragile X-associated tremor/ataxia syndrome (FXTAS). It affects older people—usually grandfathers of kids with FXS. It affects different people than those affected by FXS, and happens through a different way, but is caused by the same gene. The main symptoms include tremors (shaking), balance problems, and frequent falls  . Find out more in this patient page from American Academy of Neurology.
- YourChild: Genetic Syndromes
- YourChild: Chronic Conditions
- YourChild: Siblings of Kids with Special Needs
- YourChild: Developmental Delay has lots of information about early intervention, individualized education plans, special education, and transition to adulthood.
- Families and Fragile X Syndrome is a booklet from the National Institute of Child Health and Human Development provides comprehensive information and a list of resources.
- Fragile X Syndrome provides basic information from the National Institute of Child Health and Human Development (NICHCD)
- Genetics Home Reference: Fragile X syndrome is a useful page from the National Library of Medicine that explains the genetics of FXS, and provides links to other resources.
- The National Fragile X Foundation provides information and support, and promotes public awareness and research. They also have some Spanish language information at their site. For resource centers, parent support groups, and educational information, you can call them at 1-800-688-8765.
- The CDC Fragile X Syndrome topic page can take you to information on causes, inheritance, pre-mutation health problems, diagnosis and the CDC’s research projects. Use the right hand “Topic Contents” to navigate.
- The Fragile X Association of Michigan provides support, promotes awareness, and advances research regarding FXS.
- Carolina Fragile X Project has a wealth of information about FXS, including summaries of research findings, intervention strategies for children, services available for children with disabilities , and how to advocate for services.
- Here's some information just for kids about mental retardation, which we now call intellectual disbility.
- Fragile X Syndrome is a multimedia website from Your Genes, Your Health .
- The ARC is a national organization for people with intellectual and developmental disabilities and their families. They work to promote and improve supports and services for people with intellectual disbilities and their families. Their website has information on research, education, legislation, and advocacy.
- Fraxa is a foundation that funds FXS research. can also find out information on research, new treatments and testing. Plus has support groups, newsletters, books/products, listserv, and links to other good websites.
- Gene Tests-Gene Clinics: Fragile X Syndrome has lots of fairly academic information on inheritance, genes, genetic testing, and prenatal testing. You can register for free to get the full review.
- National Institutes of Health Fragile X Syndrome clinical trials
 Fragile X syndrome. Genetics Home Reference. Reviewed May 2007. Accessed 22 May 2007. Available from: http://ghr.nlm.nih.gov/condition=fragilexsyndrome.
 Subramony SH, Friedrich CA, Jankowiak J. Fragile X and company: finding the right diagnosis. Neurology. 2005 Jul 26;65(2):E3-4.
Written and compiled by by Kyla Boyse, R.N., Tarin Gitlin, M.D. and Elizabeth Rzepka, M.D.
Reviewed by faculty and staff at the University of Michigan.
Updated March 2010
C.S. Mott Children's Hospital