Different classes of such regulatory proteins, which you will hear more about later in connection with specific developmental control processes, are named for their characteristic structural motifs. MyoDl is an example of an HLH protein. Jun and fos are examples of bZIP proteins. At the promoter, the protein-response element complex can participate with other regulatory proteins in controlling the formation of an active transcription initiation complex. These activation steps can occur only after the chromatin structure at the locus has been opened by interactions of other proteins with the LCR as described above.
Important examples are steroid hormone receptors of the intracellular receptor superfamily.
Steroids are hydrophobic molecules that can cross cell membranes. Interaction with hormone in the cytoplasm causes them to dissociate from an inhibitory protein, thereby exposing their DNA binding domain, so that they move to the nucleus and affect gene expression by binding to specific response elements. A well-studied example, the human glucocorticoid receptor, has been shown to bind to a specific known enhancer and interact with other transcription factors to activate nearby genes. How a cell responds to a new regulatory signal may depend on its past history, i.e. on what other regulatory factors are already present.