Down syndrome: Advances in molecular biology and the neurosciences

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-fol d fewer than the number initially predicted just 10 years ago. Despite this remarkable achievement, very little is known about the mechanism(s) whereb y increased gene copy number (gene dosage) results in the characteristic ph enotype of Down syndrome. Although many of the phenotypic twits show targe individual variation. neuromotor dysfunction and cognitive and language imp airment are observed in virtually all individuals. Currently. there are no efficacious biomedical treatments for these central nervous system-associat ed impairments. To develop novel therapeutic strategies, the effects of gen e dosage imbalance need to be understood within the framework of those crit ical biological events that regulate brain organization and function.