The aims of the FAB study are:(1) to determine the efficacy and safety of low dose fluoxetine compared to placebo for reducing the frequency and severity of restricted, repetitive, and stereotypic behaviors in children and adolescents with an ASD; and (2) to explore the relationship between the effectiveness of low dose fluoxetine and the serotonin transporter (SLC6A4) genotype. The gene discussed is SLC6A4; the disease is Autistic behavior.