Fragile X syndrome (FXS) is caused by a large change, or full mutation, in the fragile X gene, located on the X chromosome. Individuals with over 200 CGG repeats have a full mutation of the FMR1 gene. A full mutation leads to loss of the fragile X protein, FMRP. It is the lack of FMRP, which is an important protein for neuron and brain development, that causes fragile X syndrome. The full mutation is inherited from a mother, who may also have a full mutation and features of fragile X syndrome, but more commonly has a smaller change, called a premutation. Women who carry a premutation are referred to as fragile X carriers and usually have never heard of the syndrome prior to being told they are a carrier.