Fragile X

17 May 2005

Fragile X syndrome is the most common inherited form of learning disability, caused by mutation of the FMR1 gene on the X-chromosome. Within the FMR1 gene is a region comprising a variable number of repeats of the sequence CGG. The normal number of CGG repeats is between 5 and about 50, whereas the mutated form of the gene has more than 200 repeats; this is referred to as a full mutation (FM). However, some individuals have between 55 and 200 CGG repeats in the FMR1 gene, and this is termed a premutation expansion (PM). Premutations can increase into the FM range when passed from mothers to their children. The frequency of FMR-1 premutations in the general population is estimated to be around 1in 250 for women and 1 in 800 for men; they are not associated with mental retardation, but around 20% of women who carry premutations experience premature ovarian failure. A recent paper [Jacquemont S et al. (2004) JAMA 291, 460-469] in the Journal of the American Medical Association reports on the prevalence of a newly identified neurodegenerative disorder affecting older male carriers of the fragile X premutation, fragile X

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