The SV40 transcriptional enhancer is composed of separate 15 to 20 base-pair-(bp)-long enhancer elements that cooperate with one another or duplicates of themselves to enhance transcription. These elements are bipartite, being composed of subunits, called enhansons, that can be duplicated or interchanged to create new enhancer elements. Enhansons differ from the enhancer elements because they are very sensitive to changes in spacing. This prototypic enhancer, therefore, contains two distinct levels of organization, each of which requires redundancy to be effective.