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Fragile X syndrome

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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The fragile X syndrome is said to be the most common heritable cause of mental retardation after Down's syndrome. It is caused by a dominant X-linked gene with a penetrance of only 50% in females.

The gene which is most commonly responsible is FMR-1 (familial mental retardation 1). The disease occurs when the expression of FMR-1 is disrupted by:

  • a large number, more than 230, of trinucleotide repeats
  • a deletion (rare)

It seems that loss of fmr-1 results in the fragile X syndrome.

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