Fragile X syndrome, the most common heritable form of mental retardation in males, affects ~1/4000 males[1]. It occurs in most ethnic and racial populations, but incidence may vary from group to group[2]. Fragile X syndrome is an X-linked genetic disorder caused by a mutation in the FMR1 gene. Female carriers may pass it on to their children. Male children are expected to have the physical and behavioral features of the condition.