Down syndrome is the most common genetic disorder associated with neurobehavioral problems, and many children with Down syndrome have clinically significant behavioral problems, including autism spectrum disorders (ASD). Preliminary research from Dr. Kaufmann's lab has shown that children with both Down syndrome and ASD have a unique profile of autistic core symptoms, including severe stereotypies (repetitive movements) which are distinct from more common movement disorders associated with Down syndrome. The present study will test and further confirm the hypothesis that the diagnosis of Down syndrome plus ASD is associated with both a unique behavioral profile and distinctive genetic characteristics. 70 boys with Down syndrome, with and without ASD, will be recruited for this study. Subjects will be evaluated for behavioral problems, including stereotypies and developmental regression, using methods of analysis which have not yet been applied to studying behavior in Down syndrome. This research may lead to improved diagnoses and treatment of Down syndrome plus ASD, and provide insight into various behavioral aspects of ASDs.