What Is Fragile X Syndrome Testing?

Fragile X syndrome (FXS) testing assesses the trinucleotide repeat expansion in the FMR1 gene. This type of test identifies FXS in more than 99% of individuals with the syndrome (both male and female) as well as carriers of FXS. FXS is one of the most common causes of:

  • Autism spectrum disorder (ASD)
  • Intellectual disability
  • Developmental delay

The American Academy of Pediatrics recommends that people with ASD be offered FXS testing along with chromosomal microarray analysis.

The FMR1 gene additionally causes a spectrum of conditions called FMR1-related disorders, dependent on the CGG repeat size and patient sex. Methylation testing can be added as a reflex test to this test if clinically indicated.

Accepted Specimens
  • Saliva via a buccal swab
  • Whole blood
  • Extracted DNA

Clinical Support for Fragile X Testing

Bionano Laboratories offers unlimited clinical support to patients and providers through access to our genetic counselors who can discuss testing capabilities, strategy, informed consent, and results.

Providers can call our genetic counselors at


FMR1 CGG repeat size is assessed using AmplideX PCR/CE FMR1 reagents. All results are reported in reference to Genome Reference Consortium Human Build 37 (also known as Human Genome 19). Reported CGG repeat sizes may vary by +/- 1 for repeats less than 60, by +/- 2–4 for repeats from 60–120, and by +/- 10 for repeats over 120.

Who should have Fragile X testing performed?

Testing should be considered for any individual with intellectual disability, developmental delay, autism spectrum disorder, or a family history of intellectual disability.

What additional tests can be performed if Fragile X testing does not provide a genetic cause?

Patients who have had uninformative FXS testing may be candidates for a chromosomal microarray test like FirstStepDx PLUS or a genetic sequencing test like NextStepDx PLUS. An evaluation or consultation by a medical geneticist (physician who specializes in genetic conditions) or genetic counselor could be useful to help determine a follow-up testing strategy.

Does this testing detect all causes of Fragile X syndrome?

This testing only detects the trinucleotide repeat expansion that makes up >99% of causes of Fragile X syndrome. Less than 1% of cases are caused by sequence variants in the FMR1, which would not be detected by this test.

How is Fragile X syndrome inherited?

The FMR1 gene is located on the X chromosome. This means that if a female is a known carrier of Fragile X syndrome, she has a 50% chance passing on the mutation to her children. Males with Fragile X syndrome cannot pass on the mutation to their sons. However, all their daughters would be carriers. Repeats between 55 and 200 (called premutations) are unstable and can increase in size as they are passed from parent to child.

Other than intellectual disability and developmental delay, what other symptoms are associated with Fragile X syndrome?

Medical problems associated with Fragile X syndrome can also include mitral valve prolapse, recurrent ear infections, seizures, strabismus, joint laxity, sleep disturbances, and gastrointestinal problems.