Fragile X syndrome is diagnosed with a blood test. A blood sample is sent to a laboratory where it is checked for the gene mutation. The test is available at most major medical centers. A health care provider, genetic counselor or the National Fragile X Foundation can provide information on testing locations.

Boys with fragile X syndrome usually are diagnosed at about 3 years of age (often at about 35 to 37 months) (7). Girls have milder symptoms and usually are diagnosed a little later (often at about 41 months) (7).

Therefore, females are affected by fragile X syndrome less frequently than males. When affected, females tend to have less severe symptoms than males. Males generally are more severely affected because they have only one X chromosome, and it contains the abnormal gene.

Crucial Information

Fragile X syndrome is the most common inherited form of intellectual disability. It affects about 1 in 4,000 males and 1 in 6,000 to 8,000 females and occurs in all racial and ethnic groups.

Fragile X syndrome is caused by an abnormality in a single gene. In 1991, a researcher supported by the March of Dimes discovered that fragile X syndrome is caused by a mutation (change) in a gene called FMR-1 located on the X chromosome (6).

Each person has 23 pairs of chromosomes, or 46 individual chromosomes. The pair of sex chromosomes (X and Y) determines whether a person is male or female. Normally, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Because females have two X chromosomes, a female who inherits one X chromosome with the abnormal FMR-1 gene still has the other unaffected X chromosome.