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. Fragile X syndrome is usually diagnosed in patients at about age 3 years. 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 (about 60% of males) and/or attention deficit hyperactivity disorder (ADHD).

One in 4000 individuals have clinical features of fragile X syndrome.  Neuropsychiatric symptoms often occur in childhood.  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. Patients have frequent otitis media in the first 3 to 4 years of life. Hernias, gastroesophageal reflux, and joint dislocations are associated conditions. Sixteen percent of males and less than 5% of females suffer from seizures. 

Adult carriers may have fragile X-associated tremor ataxia syndrome (FXTAS) or fragile X-related primary ovarian insufficiency. They also have higher rates of anxiety, depression, hypertension, migraine, and sleep apnea. Dementia develops in 50% of men with FXTAS but is seen in far fewer women.