The differentiating of neurons and other distinct cell types during embryonic development requires the selective activation or repressing of many different sets of genes. Gene expression patterns in neurons are modulated by multiple extracellular and intracellular stimuli. The transcriptional regulation of individual gene is mediated by small DNA sequences such as silencer and enhancer, and the expression pattern can be determined by the integration of the effects of a very large number of these cis-acting elements. These DNA elements either activate or repress promoter activity depending upon the nature of the transcription factors that bind to them . It is possible that there are different regulatory mechanisms of gene expression in the nerve system.