Substantial evidence suggests that chromosomal abnormalities contribute to the risk of autism. The duplication of human chromosome 15q11-q13 is known to be the most frequent cytogenetic abnormality observed in autism. We replicate this genetic abnormality in mice by using chromosome engineering to generate a 6.3 Mb duplication of the conserved linkage group on mouse chromosome 7. Mice with paternal duplication showed poor social interaction, behavioral inflexibility, abnormal ultrasonic vocalizations, and correlates of anxiety. This chromosome-engineered mouse model for autism seems to replicate various aspects of human autistic phenotypes and validates the relevance of the human chromosomal abnormality. This model will help in understanding the genetics of developmental brain disorders and thereby serve as an invaluable tool for therapeutic development.