During vertebrate CNS development, individual neurons make the decision to continue proliferating or exit and enter differentiation. These two opposing processes must be balanced else developmental abnormalities result, which can lead to dysfunction in the adult. The genes which control this process are largely unknown. The cerebellum is an excellent system to study this process because of its simple anatomy and its continued development postnatally making it more accessible for experimental analysis. Thousands of genes have been identified by the genome projects and with the advent of microarray technology it is now possible to compile a compendium of gene expression for any cellular population. A goal of this project is to generate an inventory of genes that are involved in the transition of a particular cerebellar neuron called the granule cell from a proliferative to a differentiated state by using the Affymetrix murine microarrays. Candidate genes that are differentially expressed will be tested for their function in this process by retroviral infection in vitro and a mouse transgenic tet inducible system in vivo.