Elevated serotonin levels are frequently found to be associated with autism. Serotonin levels may be regulated by the serotonin transporter (encoded by the SERT gene), which is responsible for transporting serotonin to the inside of cells. A rare mutation in the SERT gene has been previously identified in several autistic individuals. While many mutations inactivate genes, this particular mutation seems to increase rather than decrease the activity of the serotonin transporter. To examine the functional and behavioral consequences of this mutation in the SERT gene, the present study will engineer mice to carry the mutation. This study will focus on behavioral analyses to determine whether the mutation found in humans will cause autistic-like behaviors in a mouse model. These behavioral analyses will provide a basis for the future exploration of the molecular basis of autistic behaviors related to the serotonin transporter. This research should help clarify the role of variation in the serotonin transporter gene in the development of autistic-like behaviors, and may suggest novel therapies for people who have this particular mutation.