Hematopoiesis is defined by progressive changes in cytokine receptor expression and a delicate balance of intracellular activators and negative regulators of cytokine signaling. Binding of cytokines to their cognate receptors activates downstream signaling pathways and induces biologic responses. The plethora of cytokines that exhibit common structural motifs and use common receptor subunits and signaling pathways supports the notion of a highly redundant system. Mutation or deletion of components of these cytokine-signaling pathways, however, have defined remarkably specific roles for some signaling elements in hematopoiesis and shown their importance in cell survival, proliferation, differentiation, and responses to infection, as well as their potential roles in tumorigenesis.

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