Fragile X syndrome at the turn of the century.

Fragile X syndrome is not only the most common form of inherited cognitive impairment, it is also one of the most frequent single gene disorders. It is caused by a stretch of CGG-repeats within the fragile X gene, which increases in length as it is transmitted from generation to generation. Once the repeat exceeds a threshold length, no fragile X protein is produced and disease results. Since the mutation was discovered, nearly a decade of research has revealed a wealth of information regarding the fragile X gene and its possible function within the cell. The fragile X story also provides a sobering example of how much time and effort might be necessary to develop beneficial treatment through understanding gene function.