Fragile X syndrome is a syndrome of X-linked mental retardation. Boys with the syndrome may have large testicles (macroorchidism), prognathism(long face with protruding lower jaw), hypotonia(sluggish movements) and autism(brain developmental disorder), and a characteristic but variable face with large ears, long face, high-arched palate, gynecomastia(development of breasts in males), and malocclusion(misalignment of teeth). Additional abnormalities may include lordosis(back bone defect/bending), heart defect, pectus excavatum(birth defect of the chest bone), flat feet, shortening of the tubular bones of the hands, and joint laxity. Females who have one fragile chromosome and one normal X chromosome may range from normal to mild manifestations of the fragile X syndrome. The fragile X syndrome has an estimated incidence of 1 in 3600 males and 1 in 4,000--6,000 females.