Fragile X syndrome testing

Last updated: Wednesday, 11, March, 2009

MBS Online Search Page: Click here for details

Item Process
Specimen

10-20 mL blood in EDTA tube.

Method

PCR-based analysis to detect an expanded triplet repeat mutation in the regulatory region of the FMR-1 (fragile-site mental retardation) gene.

Application

Investigation of familial or sporadic mental retardation without obvious cause, in conjunction with cytogenetics, to detect the fragile X mutation.

Prenatal diagnosis for the fragile X syndrome is available - consult pathologist.

Interpretation

In normal individuals the triplet repeat region is less than 50 triplets long (150 nucleotides).

The identification of the characteristic expansion of the triplet repeat is diagnostic for the presence of the fragile X mutation.

Individuals with expansions of 60-200 triplets are unaffected but may transmit the disorder (premutation); those with expansions of approximately 200 triplets may be affected and/or transmit the disorder; males with expansions of >200 triplets will be affected.

The mothers of affected children are obligate carriers. Molecular genetic studies are indicated in other family members.