The FMR1 gene is located on the long arm of the X chromosome. At the start of this gene lies a region of DNA which varies in length from one person to another. Ordinarily, this stretch of DNA falls within a range of length that would be considered "normal".

In some people, however, this stretch of DNA is somewhat longer; this gene change is called a "premutation". Although a person who carries the premutation does not typically have symptoms of Fragile X, the stretch of DNA is prone to further expansion when it is passed from a woman to her children. When the stretch of DNA expands beyond a certain length, the gene is switched off and does not produce the protein that it is normally makes. This gene change is called a "full mutation".

A male who inherits a full mutation exhibits Fragile X syndrome because his only X chromosome contains the mutated gene. A female may not be as severely affected because each cell of her body needs to use only one of its two X chromosomes and randomly inactivates the other.

Each cell in the body contains forty-six (twenty-three pairs of) chromosomes. These chromosomes consist of genetic material (DNA) necessary for the production of proteins which lead to growth, development and physical and intellectual characteristics.

The first twenty-two pairs of chromosomes are the same in males and females. The remaining two chromosomes, X and Y, determine whether a person is male or female.

Males have only one X chromosome which is inherited from the mother. They receive a Y chromosome from the father. Females inherit two X chromosomes, one from each parent.

Fragile X syndrome is the most common inherited cause of mental impairment and the most common known cause of autism. Fragile X affects 1 in 4000 males and 1 in 6000 females of all races and ethnic groups (source Centers for Disease Control). About 1 in 259 women carry fragile X and could pass it to their children. About 1 in 800 men carry fragile X; their daughters will also be carriers.