Approximately 30 percent of boys with FXS receive a diagnosis of autism. This symptom is seen less often in girls with FXS because autism develops in cases of a full X chromosome mutation, which is rare in females. Symptoms of autism in early childhood include stereotypies such as biting, repetitive speech, hand-flapping, poor eye contact, and reduced or absent social interaction. Developmental disorders with no specific diagnosis occur in 20 to 30 percent of children with FXS.

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