Home  /  What causes FXS?

What causes fragile X syndrome?


  • Genes act as the instructions for passing on our inherited characteristics, such as eye colour and height, from parents to children.
  • Fragile X syndrome is caused by an increase in the length of one of our genes.
  • The length of the gene is divided into four categories: short, intermediate, medium and long.
  • The longer the gene, the more likely it is that the person will have symptoms of fragile X syndrome.
Length of gene associated with FXS

The gene associated with fragile X syndrome is called the FMR1 gene.

  • The FMR1 gene is located on the X chromosome and for this reason fragile X syndrome is often referred to as an X-linked condition.
  • The usual role of the FMR1 gene is to make a protein called FMRP that is important for brain development. The lengthening of the FMR1 gene means that the gene cannot function properly and the protein is not made. The lack of FMRP causes the symptoms of fragile X syndrome.

To find out more about the different gene lengths and how they expand click here.

To find out more about how fragile X syndrome is inherited and learn how it was named click here.