Fragile X-associated conditions are caused by a lengthening (expansion) in the FMR1 gene on the X chromosome.

The FMR1 gene is responsible for producing a protein called FMR1-Protein (FMRP), which is necessary for normal brain development and function.

At the beginning of the FMR1 gene there is a repeated sequence of chemical bases, known by the letters CGG. These are repeated a number of times.

Most people have less than 55 copies of this CGG repeat. Fragile X premutation carriers can have between 55 and 200 copies of the CGG repeat. People with the Fragile X full mutation (Fragile X syndrome) have over 200 copies of the CGG repeat.

DNA testing can determine how many copies of the CGG repeat a person will have.

Image credit: Women’s Health and the Fragile X Premutation, Sherman and Espinel, 2015, Emory University, Georgia, USA. 2015 https://issuu.com/fragilexaustralia/docs/final-pm-materials-12.13.15-short