What's Fragile X syndrome?
Fragile X syndrome is the most common inherited cause of intellectual disability and a common single-gene cause of autism. It is an X-linked condition caused by an expansion of a repeated stretch of DNA (a CGG repeat) in the FMR1 gene, which switches the gene off so its protein is missing. Because it is X-linked, it usually affects boys more severely than girls.
| Also indexed as | ORPHA:908, MONDO:0010383 |
|---|---|
| Features mapped | 14 |
| Treatments mapped | 0 |
| Published sources | 10 |
| Last reviewed | 2026-08-04 |
Signs and symptoms
Anxiety
Anxiety is very common and can be a major part of daily life, often showing as shyness, gaze avoidance, or distress with change. It is treatable, so it is worth raising with the care team.
Delayed speech and language development
Speech and language develop late in fragile X syndrome, and difficulties with communication are one of the earliest signs parents notice. Speech therapy and other supports started early can make a real difference.
Sleep disturbance
Disordered sleep can be part of fragile X syndrome, alongside difficulty regulating emotion, intellectual difficulties, and atypical physical development.
Attention deficit hyperactivity disorder
Attention problems and hyperactivity are among the most common features of fragile X syndrome, seen in the large majority of affected boys. They often affect learning and daily routines and can respond to the same supports and medications used for ADHD generally.
Irritability
Trouble regulating emotion can be part of fragile X syndrome, alongside disordered sleep, intellectual difficulties, and atypical physical development.
Autism
Many children with fragile X also have autism spectrum features, such as difficulty with social interaction, repetitive behaviours, and sensitivity to sensory input. Fragile X is one of the most common known single-gene contributors to autism.
Seizure
A minority of people with fragile X syndrome have seizures, usually beginning in childhood and often outgrown by adulthood. They are typically manageable with standard seizure medicines.
Moderate intellectual disability
Intellectual disability, ranging from learning difficulties to more significant disability, is the central feature. Girls are often affected more mildly than boys. Early educational and developmental support makes a real difference.
Macroorchidism
After puberty, enlarged testicles (macroorchidism) are a characteristic physical feature in affected boys. It does not usually cause problems on its own but is a helpful clue to the diagnosis.
Macrotia
Large or prominent (protruding) ears are another characteristic facial feature. Like the long face, it is one piece of a recognisable pattern, not a problem in itself.
Protruding ear
Large, protruding ears that stick out from the head are one of the most recognizable physical features of fragile X syndrome, seen together with a long face and, in males, enlarged testicles after puberty.
Long face
A long, narrow face is one of the typical physical features, usually becoming clearer with age. On its own it is subtle; it is part of a pattern rather than a standalone sign.
Mandibular prognathia
A prominent jaw is part of the characteristic facial appearance of fragile X syndrome, alongside a long, narrow face and large ears. These features tend to become more noticeable after puberty.
Joint hypermobility
Fragile X syndrome affects connective tissue, so joints are often unusually loose and flexible (hypermobile). This can show up as double-jointed fingers and flat feet, and is part of the same connective-tissue softness behind the condition's physical features.
How it is diagnosed
Fragile X syndrome
Diagnosed using: FMR1 DNA test (CGG repeat sizing and methylation).
“It is usually caused by the expansion of the CGG trinucleotide repeat (>200 repeats) in FMR1, resulting in DNA hypermethylation and gene silencing.”
Fragile X syndrome
Diagnosed using: FMR1 CGG repeat-size categories (normal, gray zone, premutation, full mutation).
“The number of CGG repeats in the FMR1 gene occurs in four distinct ranges: 2-50 (normal), 50-60 (gray zone), 60-200 (premutation), and > 200 (full mutation).”
Treatment and management
No disease-modifying treatment is established for this condition in the research mapped here. This is a stated, reviewed fact, not a missing piece of this guide.
That does not mean nothing can be done. Supportive and symptomatic care, managing specific symptoms and complications as they arise, can still matter a great deal. What is right for any individual is a conversation for their own care team.
What changes how it shows up
Carrying the genetic change is not the whole story. The factors below are described in the research mapped here as changing whether, or how strongly, the condition appears. They modulate how the genotype is expressed; they do not, on their own, cause or cure it.
FMR1 CGG full mutation with hypermethylation
Most people with fragile X syndrome carry an FMR1 gene with more than about 200 CGG repeats, the full mutation. The repeat region becomes hypermethylated, which silences the gene's switch so little or no FMRP protein is made.
Described as modulating: Fragile X syndrome.
“Most individuals with FXS have an FMR1 allele with > 200 CGG repeats (full mutation) and hypermethylation of the CpG island proximal to the repeats, which silences the gene's promoter.”
X-linked dominant inheritance
Fragile X syndrome is X-linked dominant, meaning the changed FMR1 gene sits on the X chromosome. It is linked to a wide range of features, which can include intellectual disability, autism spectrum disorder, language difficulties, macroorchidism, seizures, and anxiety.
Described as modulating: Fragile X syndrome.
“FXS is an X-linked dominant disorder associated with a wide spectrum of clinical features, including but not limited to intellectual disability, autism spectrum disorder, language deficits, macroorchidism, seizures, and anxiety.”
FMR1 premutation (55-200 CGG repeats)
A smaller FMR1 change called the premutation, between 55 and 200 CGG repeats, does not cause fragile X syndrome itself but can lead to other conditions such as fragile X-associated primary ovarian insufficiency (FXPOI) or fragile X-associated tremor/ataxia syndrome (FXTAS).
Described as modulating: Fragile X syndrome.
“Premutation (PM) populations (55-200 repeats) may present other medical issues, such as FXPOI or FXTAS.”
How to read the evidence labels
Where this comes from
This guide is built from 10 published source(s). Every claim above links back to one of them. Click any source ID to read the original on PubMed.
Take it further
Printed, source-linked documents built from this condition's graph — ready to bring to an appointment or attach to a coverage request. Every claim carries its published source, the same as this guide.