For many people with fragile X, the mutated gene that causes symptoms is active rather than silenced. Thom Leach/Science Photo LibraryFragile X syndrome is a genetic disorder caused by a mutation in a gene that lies at the tip of the X chromosome. It is linked to autism spectrum disorders. People with fragile X experience a range of symptoms that include cognitive impairment, developmental and speech delays and hyperactivity. They may also have some physical features such as large ears and foreh