Fragile X syndrome

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. It is one of the most common causes of autism and intellectual disability among boys. Individuals with this condition lack an important gene product.
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". 
In unaffected individuals, the "FMR1" gene contains 5–44 repeats of the CGG codon, most commonly 29 or 30 repeats. 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. 
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.