Mothers may be a carrier of the mutation and pass it on to their children.
It is inherited.
It is caused by mutation of a gene (FMR1) on the X-chromosome. Normally, this gene makes a protein needed for brain development. But the mutation causes a person to make less or none of the protein, which causes FXS. Guys are more likely to get it because they do not have another X chromosome like women.
The damaged gene can be passed along silently (without symptoms) for generations before a child is affected by FXS.