Pediatric Fragile X Syndrome

Trinucleotide Repeats: What are trinucleotide repeats?

DNA, the chemical that makes up our genes, contains a "code" of three-letter words known as "codons" or "trinucleotide repeats." Many genes normally contain a trinucleotide repeat which is present in several copies. But when the number of trinucleotide repeats increases to a larger than normal number of copies, the DNA is altered; therefore, the gene may not function properly or may not work at all. It is not well understood what causes a trinucleotide repeat to expand into more than the usual number of copies that should be in a gene.

Sometimes, a person may have more than the usual number of copies, but not enough to alter the function of the gene. These individuals are referred to as "premutation carriers." When they pass on these extra copies to a child, however, those extra trinucleotide repeats cause the DNA to become unstable, and the area of DNA expands even more. The result is that the child has a gene that is not functioning properly (if at all), and the child is said to have the "full mutation." An example of a trinucleotide repeat disorder is Fragile X syndrome.

About this Condition

About this Condition

What is Fragile X syndrome?

Fragile X syndrome causes intellectual disability. This can range from mild to moderate to severe, in both males and females. However, males are typically more severely affected than females. Symptoms of Fragile X syndrome in childhood are not always specific. They overlap with other disorders, such as autism, Prader-Willi syndrome, and attention deficit-hyperactivity disorder (ADHD).

Symptoms of Fragile X syndrome may include:

  • Delays in development of speech, language, and motor skills 
  • Autistic-like behavior
  • Frequent tantrums
  • Hyperactivity
  • Gaze aversion, or the inability to make and hold eye contact

The gene that causes Fragile X syndrome, called FMR1, is located on the X chromosome. Females are usually not as severely affected as males. That is because females have a normal X chromosome, in addition to the X with the mutation. The FMR1 gene normally contains fewer than 44 trinucleotide repeats. Premutation carriers have about 55 to 200 trinucleotide repeats. People with Fragile X syndrome full mutation have over 200 repeats.



The Fragile X Clinic at Children’s National offers the following treatment options for your child:
  • Medication management for common neurological and behavioral conditions
  • Recommendations on communication support and educational interventions
  • Care coordination for families and care providers of children suffering from Fragile X

Children's Team

Children's Team


Andrea Gropman

Andrea Gropman

Division Chief, Neurodevelopmental Disabilities and Neurogenetics


Fragile X Clinic

The Fragile X Clinic at Children’s National provides comprehensive evaluation and treatment to individuals and families affected by Fragile X syndrome, the most common genetic cause of intellectual disability.

Neurogenetics Program

The Neurogenetics Program team coordinates diagnostic testing, treatment, and ongoing care for children and adolescents with neurogenetic conditions, and ensures your child has access to all of the necessary specialists.

Genetics and Metabolism

With more than 4,000 visits annually and 12 physicians, Children’s National is now the largest clinical genetics program in the United States.

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