Fragile X syndrome

Diagnosis and Testing

Is there a difference in testing blood, saliva, or a biopsy for fragile X syndrome?

DNA analysis for fragile X syndrome detecting the CGG trinucleotide repeat expansion in the FMR1 gene can be done on blood, saliva, and skin biopsy samples. All samples can be tested with the same level of accuracy of detection. However, the fastest way to obtain clinical results and the preferred method by laboratories is by testing a blood sample.

Some individuals with fragile X syndrome have mosaicism, meaning they have cells in their body that have a different genetic makeup than other cells. In the case of fragile X syndrome, someone could be less severely affected because they are mosaic, meaning that only some of their body tissues (cells) have the full mutation in the FMR1 gene of greater than 200 repeats. The other body tissues may have FMR1 genes with typical repeat lengths (45 CGG repeats or less). Depending on what tissues of the body have the mutation and which do not, testing by different methods (blood, saliva, or skin biopsy) may give different genetic results.

All of this testing must be ordered by a physician or genetic counselor who can interpret the results for the patient and their family. A full list of the laboratories currently offering testing for fragile X syndrome can be found by visiting GeneTests and entering "fragile X syndrome" into the search box.

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