This is a mentored Research Career Development Award to support the career development of clinical investigators in patient-oriented research. The research project uses well established neuroscience methods (functional magnetic resonance imaging (fMRI) and genotyping) to investigate brain regions and genes involved in restricted, repetitive behavior and interest symptoms in autism spectrum disorder (ASD). Repetitive behaviors and rigid, inflexible thinking are cardinal symptoms of the disorder and prevent many otherwise high functioning people from being productive members of mainstream society. Children with and without ASD will be genotyped for a serotonin transporter gene implicated in both ASD and cognitive flexibility (the ability to switch behavioral response according to the context of the situation) and will participate in fMRI and psychological batteries that assess cognitive flexibility and ASD symptoms. Children with ASD and the risk allele for the serotonin transporter gene are predicted to make more errors on flexibility tasks and show greater deviance in the structure and function of brain regions activated. This work will help define neural and genetic contributors to restricted, repetitive behaviors in autism to help elucidate the causes of, and target interventions to treat, these core autism deficits.