Autism likely results from a combination of genetic and environmental factors, and particular genetic variations may make an individual more sensitive to environmental factors. In this study, Dr. Carpenter and colleagues will examine the interaction between a specific gene and an environmental factor in the development of autistic-like behaviors in a mouse model. The gene to be examined in the present study, the reelin gene, is a candidate genetic factor for autism, and is important for the development of the cerebral cortex and cerebellum. Mice with reduced levels of expression of this gene have neurodevelopmental defects which result in behavioral abnormalities. This research will examine the effects of an environmental factor hypothesized to be a risk factor for autism (organophosphate exposure during the prenatal period) on the development of autistic behaviors in mice which express low levels of the protein encoded by the reelin gene. It will also determine whether reduced reelin expression in combination with organophosphate exposure leads to changes in the anatomy of brain regions associated with these behaviors. This research will determine whether the reelin gene and organophosphate exposure may be risk factors for autism, and whether genetic variation in the reelin gene increases developmental sensitivity to organophosphates.