Fragile X syndrome (FXS) is the most common (known) inherited cause of intellectual disability and autism spectrum disorder. It is a genetic disorder caused by a CGG triplet repeat expansion in the FMR1 gene on the X chromosome. Typically, more than 200 CGG repeats are needed to cause disease. Males are usually more severely affected than females. Patients experience delays in language and cognitive development and typically have mild to moderate intellectual disability. Some patients meet criteria for autism and/or attention deficit hyperactivity disorder (ADHD).

Symptoms include macrocephaly, a long narrow face with prominent ears and a large jaw and forehead, high-arched palate, flat feet, hyperflexible fingers, macroorchidism postpuberty, anxiety, stereotyped movements, and hypotonia.

Adult carriers may have fragile X-associated tremor ataxia syndrome or fragile X-related primary ovarian insufficiency. They also have higher rates of anxiety, depression, hypertension, migraine, and sleep apnea.