fragile X syndrome

Last reviewed 01/2018

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.

• advice from the UK National Screening Committee on antenatal screening for Fragile X syndrome..click here