Fragile X syndrome (FXS) is a genetic syndrome that causes a range of intellectual disabilities as well as unusual physical and behavioural characteristics. Males and females can have FXS but it is more common, and often more serious, in males.[1] It is one of the most common causes of autism and intellectual disability among boys.[2][3] Individuals with this condition lack an important gene product.[1]
Technical account
Fragile X syndrome occurs as a result of a mutation of the FMR1 gene on the X chromosome. This causes an increase an increase in the number of CGG repeats in the 5' untranslated region of FMR1.[4]
In unaffected individuals, the FMR1 gene contains 5–44 repeats of the CGG codon, most commonly 29 or 30 repeats.[4][5] Between 45 and 54 repeats is considered a "grey zone". Between 55 and 200 repeats in length is described as a premutation allele. Individuals with fragile X syndrome have a full mutation of the FMR1 allele, with over 200 repeats of the CGG codon.[2][5][6]
In these individuals with a repeat expansion greater than 200, there is methylation of the CGG repeat expansion and FMR1 promoter. This silences the FMR1 gene, and its end-product is not made.
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