Fragile X syndrome

Fragile X syndrome is caused by an expanded CGG repeat
in the untranslated region of the FMR-1 gene, which results in
reduction or abolition of expression of the gene by methylation
of the gene promoter. In normal individuals, the number of
CGG repeats varies between 6 and 54 units and is stably
inherited. However, if individuals have between 55 and 200
repeats (although apparently unaffected), there is an increased
risk of the repeat region expanding further into the full
mutation range (
200 repeats) that is associated with mental
retardation.