Fragile X syndrome is a genetic condition involving changes in part of the X chromosome. It is the most common form of inherited mental retardation in males and a significant cause of mental retardation in females.

Fragile X syndrome is caused by a change in the FMR1 gene. The gene's code is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem.

Normally, the FMR1 gene makes a protein you need for for your brain to grow properly. A defect in this gene makes your body produce too little of the protein, or none at all.

Boys and girls can both be affected, but because boys have only one X chromosome, a single fragile X is likely to affect them more severely. You can have Fragile X syndrome even if your parents do not have it.

• Mental retardation
• Large testicles (macro-orchidism) after the beginning of puberty
• Large body size
• Tendency to avoid eye contact
• Hyperactive behavior
• Large forehead or ears with a prominent jaw

Family members who have fewer repeats in the FMR1 gene may not have mental retardation, but may have other problems. Women with less severe changes may have premature menopause or difficulty becoming pregnant. Both men and women may have problems with tremors and poor coordination.

The person will have a family history of Fragile X syndrome (especially a male relative).

There are very few outward signs of Fragile X syndrome in babies. Babies may have a large head circumference. Measurement of oversized testes in males who have reached puberty may also suggest the diagnosis. An experienced geneticist may note subtle differences in facial characteristics. Mental retardation is the hallmark of this condition and, in females, this may be the only sign of the problem.

A specific genetic test called polymerase chain reaction (PCR) is used to diagnose this disease. This test looks for an expanded mutation (called a triplet repeat) in the FMR1 gene.

In the past, a specific type of chromosome analysis was done. Such testing may still be available.