Fragile X syndrome is the most common cause of interited mental retardation in humans, with a frequency of approximately 1 in 1200 males and 1 in 2500 females . It is second only to Down syndrome as a genetic cause of mental retardation, which has an overall frequency of 1 in 600. These frequency estimates suggest that fragile X syndrome accounts for approximately 3% of mental retardation in males, and perhaps as much as 20% in males with IQs between 30 and 55 . The disease derives its name from the observation of a fragile site at Xq27.3 in cultured lymphocytes, fibroblasts and amniocytes .
The phenotype of the fragile X syndrome is mental retardation, usually with an IQ in the 4-70 range  and a number of dysmorphic features: long face, everted ears and large testicles [for review see ref. 5] (Fig. 1). Not every patient shows all the physical symptoms, which are generally more apparent after childhood. Macroorchidism is a common feature of fragile X syndrome in more than 90% of postpuberal males. Some patients show hyperactivity and attention deficits as well as avoidance behaviour similar to autism. Affected females generally have a less severe clinical presentation, and their IQ scores are generally higher, with typically borderline IQs or mild mental retardation.
No gross pathological abnormalities have been described in the brains of fragile X patients. Only a few post-mortem brain studies of fragile X males have been described and the information is very limited, presenting only non-specific findings such as brain atrophy, ventricular dilatation and pyramidal neurons with abnormal dendritic spines. It has been shown that the volume of the hippocampus was enlarged compared to controls , while a significantly decreased size of the posterior cerebellar vermis and increased size of the fourth ventricle was found . Using magnetic resonance imaging it was shown that fragile X patients have an increased volume of the caudate nucleus . The caudate volume is correlated with IQ and methylation status of the FMR1 gene.