A large body of literature has accumulated within the last decade concerning the fragile X syndrome, the most common cause of X-linked mental retardation. The first article of this review summarizes the peculiar genetic mechanisms and molecular biology properties (eg, unstable DNA triplet repeats), which have been characterized since the detection of the FMR-1 gene in 1991. However, the most important question concerning the function of the FMR-1 gene is still an unresolved issue and is in need of future research. The second article of this review addresses the clinical picture, neuropsychological functioning and psychopathological characteristics of pre- and full mutation carriers.