Fragile X syndrome (FXS) is a neurodevelopmental disorder associated with a single-gene mutation on the X chromosome and represents a leading hereditary cause of intellectual disability. This entry reviews what is known, across the life span, in individuals with both FXS and the fragile X premutation.

Genetic Characteristics

The fragile X mental retardation 1 (FMR1) gene is located on the long arm of the X chromosome. Because FXS is an X-linked disorder, the phenotype (observable characteristics) in males is more severe than that of females, who have an extra copy of the X chromosome to compensate, but both sexes show lower cognitive functioning when compared to mental age-matched typically developing individuals. In individuals with the fragile X full mutation (FXS), there is an excess number (defined as ...

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