The entire DNA sequence for human chromosome 21 is now complete, and it is predicted to contain only about 225 genes, which is approximately three-fold fewer than the number initially predicted just 10 years ago. Despite this remarkable achievement, very little is known about the mechanism(s) whereby increased gene copy number (gene dosage) results in the characteristic phenotype of Down syndrome. Although many of the phenotypic traits show large individual variation, neuromotor dysfunction and cognitive and language impairment are observed in virtually all individuals. Currently, there are no efficacious biomedical treatments for these central nervous system-associated impairments. To develop novel therapeutic strategies, the effects of gene dosage imbalance need to be understood within the framework of those critical biological events that regulate brain organization and function.