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Screening for fragile X syndrome could potentially reduce the incidence of mental retardation. Fragile X syndrome is a common inheritable cause of mental retardation. The odds of retardation relate to the number of repeating segments within the gene. The risk of retardation is low when a person has the premutation, which occurs when there are 50 to 200 repeats. Above 200 repeats, which is considered full mutation, the risk is high. Testing was offered to 515 people with at least a 12.5% likelihood of carrying the premutation or full mutation. Sixty-six people had the full mutation and 163 had the premutation. All 21 pregnant carriers had a chorionic villus biopsy, which identified nine embryos with the full mutation and three with the premutation. All pregnancies with the full mutation were terminated.