The normal growth, maturation, and activation of cells in most tissues requires stimulation with a defined set of growth factors. These growth factors bind cognate receptors on surface of target cells, and stimulate specific intracellular signals. Many of these signals culminate in the induction of a ligand-specific set of genes. The characterization of signaling components that mediate the induction of genes in response to IFNs has led to the identification of a new signal transduction pathway. The components of this pathway are able to rapidly transduce high fidelity signals directly from the receptor to the nucleus. Recently, many other cytokines and growth factors, including IFN-alpha, IFN-gamma, IL-6, and prolactin (PRL), have been shown to induce new genes through the same signaling paradigm.