Fragile X syndrome (FXS) is an inherited condition that can cause numerous birth defects. It involves a gene abnormality on the X chromosome that may be transmitted from a mother or father at the time of conception. 

FXS can cause cognitive impairment that ranges from learning disabilities to mental retardation and autism. It can cause behavioral problems, such as lack of attention and hyperactivity (e.g., attention-deficit hyperactivity disorder). Autistic-like symptoms, such as poor eye contact and social skills, and repetitive body movements (e.g., hand-flapping), are also common.

Physical characteristics of FXS include prominent ears, an elongated face and enlarged testicles (after puberty).

FXS affects males more often and more severely than females. It is the most common cause of mental retardation in boys, and it is a significant cause of mental retardation in females. Parents who suspect their child has FXS, or anyone with a family history of unexplained mental retardation, should contact a physician for screening.

The syndrome can be diagnosed with a DNA analysis. This test can be performed on children and adults, as well as prenatally. It can also identify people who are carriers. Carriers of FXS may not display symptoms of the syndrome, but possess the abnormal gene and are at risk of transmitting it to their offspring. They are also at risk of developing certain complications, including intentional tremors (shaking that occurs during activity but not at rest) and fertility issues.

There is no specific treatment for FXS. Therapies designed to help maximize the potential of a child with FXS include speech language therapy, occupational therapy and various medications to treat disorders associated with the syndrome. In general, the amount of care required will depend on the severity of a child’s symptoms.

Once a child has inherited the abnormal gene that causes FXS, there is no way to prevent the condition or its symptoms.