Normal nervous system function requires the generation of an enormous diversity of neurons during development. Differences in the identity and function of neurons depend upon differences in the repertoire of genes that the neuron expresses. Differential expression of these genes is controlled by intrinsic factors—the complement of transcription factors that exist within the neurons— and by extrinsic factors, signals secreted by other cells. Alterations in either of these factors have been implicated in many developmental, psychiatric and degenerative diseases. Dr. Douglas Allan is investigating how these intrinsic and extrinsic signals interact in neurons to selectively turn on the expression of different repertoires of genes in different neurons, so that the neurons attain their appropriate form and function. He is using the fruit fly, Drosophila melanogaster, as a model organism to study these mechanisms, because of the battery of powerful molecular genetic experimental tools available in this organism. Since the basic mechanisms of neuronal development are shared by fruit flies and humans, his work is relevant to understanding how human neurons develop and how disruption of these signals can cause disease.