Fragile X syndrome or FXS is a genetic condition. The approximate prevalence of FXS is one in 3,600 to 4,000 males and one in 4,000 to 6,000 females. FXS can lead to varying degrees of intellectual and behavioral challenges and a range of physical characteristics. Mutations in the FMR1 gene results in deficient production of the protein FMRP, which in turn causes FXS. Individuals can have different forms of the mutated gene. The severity of symptoms depends on how much the mutated gene differs from a healthy FMR1 gene.


Humans have 23 chromosome pairs inherited from both parents. Chromosomes carry genes, so each parent also contributes half of a child's genes. The 23rd chromosome pair contains X and Y sex chromosomes. Females have two X chromosomes (XX), while males have an X and Y (XY). Mothers always contribute an X chromosome to offspring so the father's contribution of an X or Y determines gender. The FMR1 gene is on the X chromosome.

Fragile X syndrome

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FXS is more common and severe in males because they have only one X chromosome. Females have two X chromosomes, which means they have two different FMR1 genes. A healthy FMR1 gene can counteract a mutated gene, so many women carrying the FXS mutation do not show any symptoms, but their sons might if they inherit the mutation instead of the healthy gene. FXS also tends to worsen with each generation because the different mutations or permutations become gradually more severe.

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Physical Characteristics in Males

The physical characteristics of FXS may not be apparent in infants or young children. Individuals rarely exhibit every trait, and some men have no physical indicators of FSX at all. Physical traits include a long face, weak muscle tone, highly arched palate in the roof of the mouth and overly large ears. Connective tissue abnormalities manifest as flat feet, hyper-flexible joints, double-jointed fingers, and soft skin. Enlarged testes, known as macroorchidism, develop during puberty.

Fragile X syndrome children

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Behavioral Symptoms

Behavioral issues are present in over half of children with fragile X syndrome, and they are not in proportion to intellectual challenges. This means the same level of cognitive impairment in children with other developmental disorders may not cause such severe behavioral issues. Behavioral symptoms occur in clusters. Symptoms similar to those associated with attention-deficit disorders are present in 80% of people with FXS. These include hyperactivity, inattentiveness, restlessness, and impulsive decisions. Aggressive and self-harming behaviors are common and related to rapidly changing emotions, irritability, and the inability to regulate anger.

Fragile X syndrome chromosome

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Effects in Adulthood

Boys with FXS often become more mild-mannered as they approach adulthood. They develop a good sense of humor and become very hard-working and determined to accomplish tasks. Women with a full mutation and symptomatic FXS exhibit anxiety-related symptoms such as obsessive-compulsive disorder, shyness, social avoidance, social anxiety, mood swings, and depression. Men over 50 with FXS sometimes develop tremors, rapid onset of dementia, incontinence, and symptoms associated with movement disorders.

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Approximately 30 percent of boys with FXS receive a diagnosis of autism. This symptom is seen less often in girls with FXS because autism develops in cases of a full X chromosome mutation, which is rare in females. Symptoms of autism in early childhood include stereotypies such as biting, repetitive speech, hand-flapping, poor eye contact, and reduced or absent social interaction. Developmental disorders with no specific diagnosis occur in 20 to 30 percent of children with FXS.

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Doctors can confirm a definitive diagnosis of fragile X syndrome through molecular genetic testing of the FMR1 gene. They will most often do this test when a parent or physician notes a combination of factors such as developmental delays and family history. Prenatal testing is also possible. Women carrying the mutated gene as a recessive allele do not have any symptoms, but ovarian failure at a young age is much more common for women carrying the FMR1 gene mutation than in the general population. Obstetrics doctors may recommend young women having trouble conceiving undergo genetic testing as a precaution to see if they are carriers.

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Cognitive Symptoms

Intellectual impairment is directly related to the severity of the mutated gene and almost always affects males with FXS. Females with FXS rarely have more than mild intellectual impairment and often don't exhibit any cognitive symptoms. Specialized education and assistance from a young age is the best way to help children with cognitive challenges and learning disabilities reach their full potential, and early intervention can lead to a full life.

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Adult Support

Therapy is the best way to support independence as an adult for those with FXS; this can improve interpersonal interactions and ensure co-occurring mental health disorders are addressed. Marriage, family, work, and an active social life are attainable goals, even if genetic counseling advises against children.

Fragile X syndrome


Managing Symptoms

Developmental disorder assessment tools are helpful for children and families managing FXS. Therapy is supportive and tailored to symptoms. Medication can supplement speech and occupational therapy, behavioral intervention, and special education. Anxiety and obsessive-compulsive or self-harming behaviors often require specialized treatment programs and medication. 

FXS Fragile X syndrome

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