fragile X syndrome
a genetic condition that causes a range of developmental problems, including learning disabilities and cognitive impairment. The disorder is so named because of alterations in the FMR1 gene, on the arm of the X chromosome, that abnormally expand and destabilize it. Males with fragile X syndrome have characteristic physical features that become more apparent with age, such as large ears, prominent jaw and forehead, a long and narrow face, and enlarged testicles. Both males and females with fragile X may exhibit hyperactivity, social anxiety, and attention deficits; some males also show autistic behavior.