Answer: a, d Although most hormone and other messenger receptors are extracellular, intracellular receptors have been identified. The hormone messengers involved for these receptors are primarily steroid and thyroid hormones and are lipophilic. By virtue of their hydrophobic nature, they are able to readily penetrate the lipid portion of the cell membrane. Receptors for these hormones exist intracellularly in the cytoplasm or in the nucleus and generally act as regulators of gene expression. These hydrophobic signaling molecules exist in plasma bound to protein, so that the concentration of this class of regulators does not fluctuate rapidly in plasma and their actions are generally slower in onset and more prolonged than those of water-soluble class. Some types of steroid receptors, particularly for glucocorticoids, are located in the cytosol in the inactive state. Once the ligand binds, the receptor undergoes a conformational change, termed activation. This allows cytoplasmic receptors to move into the nucleus and bind to DNA. Receptors already in the nucleus increase their affinity for DNA. In the case of glucocorticoid receptors and probably others of this class, the inactive receptor is associated with another protein, the heat-shock protein. They block the DNA-binding domain of the receptor. Activation involves the dissociation of the inhibitor protein