What causes Fragile X syndrome?

Fragile X results from a change or mutation in the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene, which is found on the X chromosome. The gene normally makes a protein, called FMRP, that is important for creating and maintaining connections between cells in the brain and nervous system. The mutation causes the body to make only a little bit or none of the protein, which often causes the symptoms of Fragile X.

Not everyone with the mutated FMR1 gene has symptoms of Fragile X syndrome, because the body may still be able to make FMRP. A few things affect how much FMRP the body can make:

  • The size of the mutation. Some people have a smaller mutation (a lower number of repeats) in their FMR1 gene, while others have big mutations (a large number of repeats) in the gene. If the mutation is small, the body may be able to make some of the protein. Having the protein available makes the symptoms milder.
  • The number of cells that have the mutation. Because not every cell in the body is exactly the same, some cells might have the FMR1 mutation while others do not. This situation is called mosaicism. If the mutation is in most of the body’s cells, the person will probably have symptoms of Fragile X syndrome. If the mutation is in only some of the cells, the person might not have any symptoms at all or only mild symptoms.
  • Being female. Females have two X chromosomes (XX), while males have only one. In females, if the FMR1 gene on one X chromosome has the mutation, the FMR1 gene on the other X chromosome might not have the mutation. Even if one of the female’s genes has a very large mutation, the body can usually make at least some FMRP, leading to milder symptoms.
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