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Paediatric Laboratory Medicine

Fragile X Syndrome: FMR1 Trinucleotide Repeat Analysis

Clinical Significance

Fragile X syndrome is an X-linked disorder with variable expression in carrier males and females. It is more severe in males than in females, although females are more likely to transmit the disease to their children. Affected males usually have mental retardation, behaviour problems and speech and language delays. They may also have a number of physical characteristics of the syndrome. Approximately 35 per cent of females who carry the mutation are mentally retarded to varying degrees, although usually less than affected males. Many show emotional and problem solving difficulties as well as learning disabilities.

The gene responsible for Fragile X Syndrome is called FMR-1 and is located on the X chromosome. The normal gene contains a three base pair sequence, which is repeated on each X chromosome (called a CGG repeat). Although variable in the general population, the number of repeats is usually inherited without change from generation to generation. The principal mutation causing Fragile X Syndrome is an expansion of the CGG repeat sequence within the FMR-1 gene. The mutation is also associated with abnormal methylation of the FMR-1 gene. Methylation interferes with normal FMR-1 gene expression, resulting in the Fragile X phenotype.

In normal individuals the number of CGG repeats within the FMR-1 gene ranges in size from six to 44 repeats, whereas patients affected with the Fragile X Syndrome show expansion ranges greater than 200 repeats (full mutation). Expansions in the number of repeats between 55 to 200 are called premutations and usually do not result in any symptoms of Fragile X in females or in males (called 'carrier females' and 'transmitting males'). However, premutations are unstable and may expand further when transmitted to offspring, resulting in a full mutation and Fragile X Syndrome. Expansions of 45-54 repeats are considered intermediate. Alleles in this range are stable in some families but unstable in others and may lead to premutations in subsequent generations. In approximately one per cent of Fragile X cases, point mutations or deletions, rather than expansions, in the FMR-1 gene are the cause of the syndrome.

Test Name

Fragile X Syndrome: FMR1 Trinucleotide Repeat Analysis

Alternate Name/ Synonym

  • FRAXA syndrome
  • Fragile X Tremor Ataxia syndrome
  • FMR1-related primary ovarian insufficiency

Gene Name

  • FMR1

Alternate Gene Name


Test Code



Molecular Genetics


FMR1 5'UTR trinucleotide (CGG) repeat analysis via PCR

External Proficiency Testing


Turn Around Time

2 weeks (Prenatal samples), 2-3 weeks (Pregnancy/Urgent samples), 4-6 weeks (Routine)


Fragile X mental retardation protein 1


Fragile X Syndrome

Specimen Type

Blood; gDNA. 

For details about specimen requirements, please refer to: Specimen Type & Requirements (PDF).

Minimum Specimen Requirements

5-10 mL EDTA or ACD 0.5 mL EDTA (neonate); minimum 10 ug in 100 uL low TE (pH8.0)


Room Temperature

Special Requirements

Special Instructions for Genome Diagnostics Samples

If sample shipment >48 hours, ship on ice.

Approval is not required

CPT Codes


Shipping and Contact Information

The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642
Toronto, ON
M5G 2G3
Phone: 416-813-7200
Phone: 1-855-381-3212


Interpretation is provided in report

Information Sheet: Fragile X Syndrome