Cellular immediate-early genes (IEGs) share a close structural homology with some viral oncogenes. In the eukaryotic genome these counterparts of oncogenes have been termed proto-oncogenes. Recent advances in cellular biology have identified the activation and deactivation of IEGs as molecular mechanisms to control regulated and deregulated growth, cellular differentiation and development. In neurobiology IEGs are believed to be involved in the neuron’s ability to convert short-term synaptic stimulation into long-lasting responses and thus contributing to the adaptive alterations involved in neuronal plasticity (Evan 1991; Goelet et al. 1986; for review see Morgan and Curran 1991). IEGs may be viewed as “third messengers” in a stimulus-transcription cascade transferring extracellular information via second messenger systems, such as calcium, into changes in target-gene transcription, thereby changing the phenotype of neurons. IEGs which encode proteins with a leucine zipper structure have to dimerize to function. While Jun proteins can form homo- as well as heterodimers, members of the Fos family require dimerization with Jun proteins. The various IEG complexes possess different binding affinities for DNA consensus sequences (AP-1 site) and variable transcriptional activities. The zinc-finger protein Krox-24, also termed NGFI-A, Egr-1, Zif/268, can achieve transcriptional activation in a monomeric fashion.