These are the transcription factors that are responsible for differential regulation of transcription. 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. 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. 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.