Hypothetical structure of the IFNβ enhanceosome. The IFNβ enhancer is referred to as an enhanceosome because, unlike other enhancers, it requires accurate spacing and helical phasing of the DNA between several protein-binding sites. This requirement indicates that a specific tertiary structure is formed with the various regulatory proteins. HMG proteins, while not part of the completed complex, facilitate enhanceosome formation by helping to bend the DNA (see Figure 21-6). Individual regulatory proteins in the complex are shown in different colors.