Fragile X syndrome

Fragile X syndrome is the most common identifiable cause of inherited intellectual disability and autism spectrum conditions. It arises from changes on the X chromosome in a specific gene that normally makes a protein necessary for brain development. Boys are usually more severely affected than girls as they have only one X chromosome. Girls have a second X chromosome, which can compensate for problems with the faulty one.  However some girls can be quite severely affected while some boys are only mildly affected.

In this article

What are the symptoms of fragile X syndrome?

The main feature is intellectual disability. This can range from very minor, so that the person has a normal IQ and shows no signs of fragile X syndrome though may experience difficulties with specific areas such as mathematics and social abilities, to severe intellectual difficulties. How badly someone is affected depends to a degree on the amount of genetic change on their X chromosome.

Other problems include delayed and distorted speech and language development. There can be difficulties with the social use of language and speech. There may also be repetitive behaviour, attention deficits and overactivity. In some individuals there may be evidence of autistic-like features (see entry Autism Spectrum conditions), such as repetitive speech, poor eye contact, hand flapping, social anxiety, abnormal shyness and an insistence on routine.

Associated physical features include a relatively large head, a long face with prominent ears, largish jaw and double-jointedness. However, these features are rarely obvious. Additionally, thirty per cent of people with fragile X syndrome develop epilepsy.

What are the causes of fragile X syndrome?

The cause is an abnormal DNA expansion just above the tip of the X chromosome’s long arm. The expansion consists of small repeats of a DNA sequence, which are unstable and can get larger across generations. This means that a child can be more affected than the parent from which they inherited the changed X chromosome.

How is fragile X syndrome diagnosed?

Diagnosis is by blood test, using DNA analysis to test for the change in the gene.

How is fragile X syndrome treated?

There is no cure for fragile X syndrome. Psychological and educational support for children and their families are vital to help them reach their best potential and to maximise their quality of life.

For children with delayed speech and language development, specialised help from a speech and language therapist is important. Sensory issues and coordination difficulties may be helped by an occupational therapist. Hyperactivity and impulsivity may be alleviated by behavioural therapy and in some cases medication. Behavioural therapy can help children who have problems developing relationships, need assistance in developing social skills and need help coping with stressful situations.

Some children can be educated in mainstream schools, with support. However, some with more severe intellectual disability, or complicating factors like ASD or ADHD may need special schooling.

Adults with fragile X syndrome may require support with daily living tasks and may benefit from assisted living arrangements. This is dependent on the degree of intellectual disability.

Inheritance patterns and prenatal diagnosis

Inheritance patterns
Fragile X syndrome is inherited in an autosomal recessive manner. Some people are carriers of the condition meaning that they carry an affected X chromosome but do not experience adverse consequences. However, the genetics is more complicated as some people only have a small change, called a permutation, in the gene. They too can experience developmental and psychological challenges but these are usually less marked than when there is a ‘full mutation’ of the gene. A premutation makes the gene unstable and at risk of developing into a full mutation when passed onto the next generation.

Prenatal diagnosis
Prenatal diagnosis is possible by chorionic villus sampling where a small piece of tissue from the placenta is analysed genetically. Fetal blood sampling is also possible.

Is there support for people with fragile X syndrome and their families?

If your child is affected by a medical condition or disability, we can help. Call our freephone helpline on 0808 808 3555 to get information, support and advice. We also offer emotional support for parents via our Listening Ear service.

We have a range of parent guides on aspects of caring for a disabled child in our resource library. You may also find our Early Years Support useful, which contains links to parent carer workshops and help for families going through the diagnosis process.

We’ve listed some support groups below and you can also meet other parents online in our closed Facebook group.

Fragile X Society

Tel: 01371 875 100

The Society is a Registered Charity in England and Wales No. 1127861. It provides information and support to families affected by fragile X. The Society creates a network of families through an online community and support groups, and provides information in multiple languages, including Gujarati, Hindi and Urdu.

Group details last reviewed March 2018.


Medical text written April 2016 by Professor J Turk. Last updated March 2018 by Professor J Turk,  Professor of Developmental Psychiatry, Institute of Psychiatry, University of London and Consultant Child and Adolescent Psychiatrist, Child and Adolescent Mental Health Neurodevelopmental Services, South London and Maudsley NHS Foundation Trust, London, UK.

Although great care has been taken in the compilation and preparation of all entries to ensure accuracy, we cannot accept responsibility for any errors or omissions. Any medical information is provided for education/information purposes and is not designed to replace medical advice by a qualified medical professional.

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