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'''Fragile X syndrome''' is the second most frequent genetic cause of mental retardation, after [[Down syndrome]]. The disorder is one of a group of diseases that results from an unusual kind of mutation: an expansion of a repeating sequence of three letters of the DNA code, called a [[triplet repeat]] or [[trinucleotide repeat]].

'''Fragile X syndrome''' is the second most frequent genetic cause of mental retardation, after [[Downs syndrome]]. The disorder is one of a group of diseases that results from an unusual kind of mutation: an expansion of a repeating sequence of three letters of the DNA code, called a [[triplet repeat]] or [[trinucleotide repeat]].

In Fragile X, the repeating triplet is CGG, cytosine-guanine-guanine, in a gene called FMR1 on the X chromosome. There are ordinarily fewer than 55 copies of the repeat. When the number of repeats exceeds 200, the expansion is called a "full mutation" and expression of the FMR1 gene is shut off, causing the fragile X syndrome.

In Fragile X, the repeating triplet is CGG, cytosine-guanine-guanine, in a gene called FMR1 on the X chromosome. There are ordinarily fewer than 55 copies of the repeat. When the number of repeats exceeds 200, the expansion is called a "full mutation" and expression of the FMR1 gene is shut off, causing the fragile X syndrome.