Note: Figure 7.17 demonstrates a eukaryotic example of an enhancer. Signal molecules, such as cyclic AMP (cAMP), cortisol, and estrogen, bind to specific receptors. For the examples given, these receptors are cyclic AMP response element-binding protein (CREB), the glucocorticoid (cortisol) receptor, and the estrogen receptor, respectively; all are transcription factors that bind to their respective response elements within the enhancer. Other proteins are involved in this process, but are outside the scope of the MCAT. Note that the large distance between the enhancer and promoter regions for a given gene means that DNA often must bend into a hairpin loop to bring these elements together spatially.