fragile X chromosome syndrome

n. A genetic defect in which the long arm of the X chromosome often breaks off, leading to mental retardation. This syndrome is more common in males and is the second leading cause of mental retardation among males and often accompanied by attention deficit- hyperactivity syndrome or autism. Although there are no definitive physical characteristics for the syndrome, it is associated with large testes, large ears, long face, and malocclusion of the teeth. Also called the Martin- Bell syndrome.