Syndrome, fragile X: The most common heritable form of intellectual disability, occurring in about one in five thousand males and a smaller percentage of females. Characteristics of Fragile X syndrome in boys include prominent or long ears, a long face, delayed speech, large testes (macroorchidism), hyperactivity, tactile defensiveness, gross motor delays, and autistic-like behaviors. Much less is known about girls with Fragile X syndrome. Only about half of all females who carry the genetic mutation have symptoms themselves. Of those, half are of normal intelligence, and only one-fourth have an IQ under seventy. Few Fragile X girls have autistic symptoms, although they tend to be shy and quiet. Fragile X syndrome is due to a dynamic mutation (a trinucleotide repeat) at an inherited fragile site on the X chromosome, and so is an X-linked disorder. Because the mutation is dynamic, it can change in length and hence in severity from generation to generation, from person to person, and even within a given person. Fragile X syndrome is diagnosed with a genetic test. Also known as FRAXA (as is the Fragile X chromosome itself) and Martin-Bell syndrome.