Multicellular organisms require a means of intracellular communication to organize and develop the complex body plan that occurs during embryogenesis and then for cell and organ systems to access and respond to an ever changing environmental milieu. Mediators of this constant exchange of information are growth factors, neurotransmmitters, peptide and protein hormones which bind to cell surface receptors and transduce their signals from the extracellular space to the intracellular compartment. Via multiple signaling pathways, receptors of this general class affect growth, development and differentiation. Smaller hydrophobic signaling molecules, such as steroids and non-steroid hormones, vitamins and metabolic mediators interact with a large family of nuclear receptors.