After Down syndrome, the second most frequent genetic cause of mental retardation. The disorder is one of a group of diseases that results from an unusual kind of mutation: 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 on the X chromosome. The larger the number of repeats they possess, the more likely patients are to be seriously impaired. People who possess just a few repeats are carriers but often not affected.

Dr. Robert Nussbaum, of the National Human Genome Research Institute's Laboratory of Genetic Disease Research, defines Fragile X syndrome.

Related terms: cell, chromosome, gene, nucleotide, sex chromosome

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