Fragile X syndrome


What genetic change causes fragile X syndrome?

Fragile X syndrome is caused by a CGG trinucleotide ("tri" meaning "three", "nucleotide" meaning "nucleotide base") repeat expansion of greater than 200 repeats in the promoter region of the fragile X mental retardation 1 (FMR1) gene on the X chromosome and abnormal hypermethylation (turning off of the gene). In very rare cases (1% or less), gene mutations at locations other than the CGG repeat in the FMR1 gene have been described.

Many different genetic testing laboratories are able to analyze the FMR1 gene. To learn what laboratories are currently offering testing, please visit GeneTests and enter "fragile X syndrome" into the search bar.

Sherman S, Pletcher BA, Driscoll DA. Fragile X syndrome: Diagnostic and carrier testing. Genetics in Medicine. 2005;7(8):584-587. doi:10.1097/01.GIM.0000182468.22666.dd.

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