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.