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Genetic and Rare Diseases Information Center (GARD)

Other Names for this Disease
  • Fra(X) syndrome
  • FRAXA syndrome
  • FXS
  • Marker X syndrome
  • Martin-Bell syndrome
More Names
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What causes fragile X syndrome?

Mutations (changes) in the FMR1 gene cause fragile X syndrome. This gene carries instructions to make a protein called the fragile X mental retardation 1 protein. In most cases of fragile X syndrome, a specific DNA segment, known as the CGG triplet repeat, is expanded within the FMR1 gene. Normally, this DNA segment is repeated from 5 to about 40 times. In people with fragile X syndrome, however, the CGG segment is repeated more than 200 times. The abnormally expanded CGG segment turns off (silences) the FMR1 gene, which in turn disrupts nervous system functions and leads to the signs and symptoms of fragile X syndrome. In a small percentage of cases, other types of mutations cause fragile X syndrome. These mutations delete part or all of the FMR1 gene, or change one of the building blocks (amino acids) used to make the gene's protein. As a result, no protein is produced, or the protein's function is impaired because its size or shape is altered.[1]

Males and females with 55 to 200 repeats of the CGG segment are said to have an FMR1 gene premutation. Most people with a premutation are intellectually normal. In some cases, however, individuals with a premutation have lower than normal amounts of the fragile X mental retardation 1 protein. As a result, they may have mild versions of the physical features seen in fragile X syndrome (such as prominent ears) and may experience emotional problems such as anxiety or depression. Some children with a premutation may have learning disabilities or autistic-like behavior. About 20 percent of women with a premutation have premature ovarian failure, in which menstrual periods stop by age 40. Men, and some women, with a premutation have an increased risk of developing a disorder known as fragile X-associated tremor/ataxia syndrome (FXTAS). This disorder is characterized by progressive problems with movement (ataxia), tremor, memory loss, loss of sensation in the lower extremities (peripheral neuropathy), and mental and behavioral changes.[1]
Last updated: 6/15/2011

  1. Fragile X syndrome. Genetics Home Reference. May 2007; Accessed 6/15/2011.