Fragile X Syndrome is the most common cause of mental retardation, affecting approximately 1 in 1500 males and 1 in 3000 females (figures given vary; these are approximations). It is caused by a mutation of a region of the X chromosome, and specifically, an excessively large number of repetitions of a specific gene on that chromosome.

Because men receive only one copy of the X chromosome, most (all?) males with the mutation exhibit at least some symptoms of the syndrome, while only approximately 30 percent of females will exhibit significant symptoms, with the rest being carriers of the mutation. Aside from mental retardation, the most obvious indicators include physical differences and behaviors commonly associated with autism. Of the former, the most readily visible are an elongated face and large or protruding ears, but others are also frequently present. Of the latter, behavioral stereotypy and atypical social development are the most frequently observed.

Because the underlying cause is genetic, there is no cure. The syndrome can be treated through behavioral therapy, special education, and where necessary, treatment of physical abnormalities. Persons with Fragile X in their family histories are advised to seek genetic counseling to assess the likelihood of having children who are affected, and how severe any impairments may be in affected descendants.