Fragile X syndrome, also called Martin-Bell syndrome, is an inherited genetic disorder that can cause intellectual and developmental disabilities, as well as social and behavioral problems.

Fragile X syndrome is the most common cause of inherited intellectual disability, affecting an estimated 1.4 in 10,000 males and 0.9 in 10,000 females, according to the Centers for Disease Control and Prevention (CDC).

People who have fragile X syndrome have learning and developmental disabilities, which can vary from mild to severe. It is a chronic condition that can significantly impair a person’s ability to live independently.

Keep reading to learn more about fragile X syndrome, including its causes, symptoms, and diagnosis.

Boy with Fragile X syndrome playing with toyShare on Pinterest
A person with fragile X syndrome may have intellectual and developmental disabilities.
Image credit: Peter Saxon, 2014.

Genetic disorders, including fragile X syndrome, occur when a person inherits one or more mutated genes from their parents. Genes contain segments of DNA that carry instructions for making proteins and molecules.

People have 23 pairs of chromosomes, which contain between 20,000 and 25,000 genes in total. The first 22 pairs are called autosomes. Autosomes look relatively similar in most people.

The last pair of chromosomes — either X or Y — determines a person’s sex. Males have one X and one Y chromosome, while females have two X chromosomes.

Fragile X syndrome occurs as a result of mutations in the FMR1 gene on the X chromosome. The FMR1 gene provides instructions for making the FMRP protein. FMRP plays a vital role in the development of synapses, which serve as communication junctions between nerve cells.

FMRP also carries messenger RNA (mRNA) molecules that hold information for the production of proteins.

The specific mutation occurs in a portion of DNA on the FMR1 gene called the CGG triplet repeat region. Usually, the CGG DNA segment repeats between 5 and 40 times. However, people with fragile X syndrome have CGG segments that repeat more than 200 times.

People who have expanded CGG segments do not always develop fragile X syndrome. Experts consider CGG repeats between 45 and 54 to be intermediate expansions. Individuals who have 55–200 repeats have a premutation, which can expand further into a full FMR1 mutation in future generations.

An abnormally long, or expanded, CGG segment inactivates the FMR1 gene, which leads to a significant reduction in or total absence of FMRP.

Fragile X syndrome can cause mild-to-severe intellectual and developmental disabilities. Some people also experience social and behavioral problems.

Fragile X syndrome usually causes moderate intellectual disability in males and mild or no intellectual disability in females. Intellectual disability refers to limitations in cognitive functioning and learning abilities that affect children under the age of 18 years.

People with fragile X syndrome may experience the following symptoms:

  • intellectual and learning disabilities, including having trouble learning a new skill or retaining new information
  • developmental delays, such as sitting up, crawling, walking, or learning to speak later than expected
  • social or behavioral problems, such as avoiding eye contact with others, aggression, shyness, or difficulty understanding body language
  • hyperactivity, depression, or anxiety
  • difficulty sleeping

People who have fragile X syndrome may develop the following physical features:

  • enlarged ears
  • elongated face
  • protruding chin
  • prominent forehead
  • large head (macrocephaly)
  • cleft palate
  • flat feet
  • crossed eyes (strabismus)
  • large testicles (macroorchidism), in males

There is no cure for fragile X syndrome, but treatment services can help teach people valuable language, learning, and social skills. Early intervention services help infants and young children with developmental delays build the following skills:

  • physical movements, such as crawling and walking
  • cognitive abilities and problem-solving
  • communication skills, such as listening and speaking
  • social skills, including playing with others
  • essential daily functions, such as getting dressed and eating

People who have a child with developmental disabilities can contact their pediatrician for more information about early interventions. Parents and caregivers can always speak with a pediatrician if they have concerns about their child’s development.

A doctor may prescribe medication or recommend cognitive behavioral therapy to treat behavioral or mood disorders, such as attention deficit hyperactivity disorder (ADHD) or anxiety.

Treatment often involves several people, including caregivers, healthcare professionals, speech and physical therapists, and teachers and coaches.

Having a community of individuals willing to work with and support children with fragile X syndrome can help improve a child’s confidence and overall quality of life.

A doctor can diagnose fragile X syndrome by testing a person’s DNA from a blood sample.

Parents or caregivers may wish to consider seeking a test for their child if they show signs of developmental or intellectual disabilities or if they have a family history of fragile X syndrome.

Doctors can also test people for CGG premutations. The results of a DNA test may affect other family members, so people may benefit from having genetic counseling as well.

Fragile X syndrome is a genetic condition that can cause mild-to-severe intellectual and developmental disabilities that may inhibit a person’s ability to live independently.

Fragile X syndrome is a lifelong condition with no known cure.

Healthcare specialists and other community members can help support people with fragile X syndrome.