Fragile X syndrome: The most common heritable form of mental retardation. Fragile X syndrome is due to a dynamic mutation (a trinucleotide repeat) at an inherited fragile site on the X chromosome; therefore, it is an X-linked disorder. Characteristics of fragile X syndrome in boys may include, in addition to mental impairment, prominent or long ears, a long face, delayed speech, hyperextensible joints, hyperactivity, tactile defensiveness, gross motor delays, and autistic-like behaviors. Boys are typically more severely affected than girls with fragile X syndrome. Only about half of all females who carry the genetic mutation for fragile X syndrome have symptoms themselves. Because the mutation is dynamic, it can vary in length and hence in severity of its effects from generation to generation, from person to person, and even within a given individual. The diagnosis of the syndrome is confirmed by molecular genetic testing. Also known as FRAXA (as is the fragile X chromosome itself) and Martin-Bell syndrome.