The main feature is intellectual disability. This can range from very minor, so that the person has a normal IQ and shows no signs of fragile X syndrome though may experience difficulties with specific areas such as mathematics and social abilities, to severe intellectual difficulties. How badly someone is affected depends to a degree on the amount of genetic change on their X chromosome.
Other problems include delayed and distorted speech and language development. There can be difficulties with the social use of language and speech. There may also be repetitive behaviour, attention deficits and overactivity. In some individuals there may be evidence of autistic-like features (see entry Autism Spectrum Disorders, including Asperger syndrome), such as repetitive speech, poor eye contact, hand flapping, social anxiety, abnormal shyness and an insistence on routine.
Associated physical features include a relatively large head, a long face with prominent ears, largish jaw and double-jointedness. However, these features are rarely obvious. Additionally, thirty per cent of people with fragile X syndrome develop epilepsy.