Cytokines control growth, differentiation, and function of cells of lymphocytic, hemopoietic systems and provide a pertinent model to study intercellular communications and intercellular signal networks. Cytokines have key properties that shape the nature of the network. 1) A single cytokine has multiple biological activities (pleiotropy). 2) Multiple cytokines have overlapping activities(redundancy). 3) A single cell interacts with multiple cytokines with seemingly identical responses(cross-talk). Gene cloning of cytokines and their receptors provide knowledge to understand molecular mechanisms of these phenomena. Within recent studies of modification of the cytoplasmic region of the cytokine receptor or the signaling molecules, it will become possible to apply these molecules for therapeutic purposes.