A genetic disorder that causes mental retardation.

Fragile X syndrome is a genetic disorder that occurs in all ethnic groups, and one out of 700 pregnant women carries the fragile X defect. In 1992, a new test was announced for determining whether a woman carries the fragile X genetic defect. When a woman learns that she carries this defect, she may then elect to undergo amniocentesis or chorionic villus tests to determine whether her fetus also carries the fragile X defect. The incidence of fragile X syndrome is estimated to be approximately one in every 2,000 to 3,000 births; the estimates include one per 1,500 males and one per 2,500 females.

Fragile X appears to be caused by an abnormal number of repeats of a genetic sequence on a segment of the X chromosome. (It was initially characterized as the Martin-Bell syndrome.) Because fragile X has been observed in all...