Fragile X syndrome ( is characterized by moderate to severe mental retardation, macroorchidism, and distinct facial features, including long face, large ears, and prominent jaw. In most cases, the disorder is caused by the unstable expansion of a CGG repeat in the FMR1 gene and abnormal methylation, which results in suppression of FMR1 transcription and decreased protein levels in the brain (Devys et al., 1993).


Fragile X syndrome accounts for about one-half of cases of X-linked mental retardation and is the second most common cause of mental impairment after trisomy 21 (190685) (Rousseau et al., 1995).

McCabe et al. (1999) summarized the proceedings of a workshop on the fragile X syndrome held in December 1998.

Jacquemont et al. (2007) provided a review of fragile X syndrome, which they characterized as a neurodevelopmental disorder, and FXTAS, which they characterized as a neurodegenerative disorder.