Hematopoiesis is regulated through the binding of cytokines to receptors of the cytokine receptor superfamily. Although lacking catalytic domains, members of the cytokine receptor superfamily mediate ligand-dependent activation of protein tyrosine phosphorylation through their association and activation of members of the Janus kinase (Jak) family of protein tyrosine kinases. The activated Jaks phosphorylate the receptors which creates docking sites for SH2-containing signaling proteins which are tyrosine phosphorylated following their association with the complex. Among the substrates of tyrosine phosphorylation are members of the signal transducers and activators of the transcription family of proteins (Stats). Various cytokines induce the tyrosine phosphorylation and activation of one or more of the seven family members. The pattern of Stat activation provides a level of cytokine individuality that is not observed in the activation of other signaling pathways. The role of various Stats in the biological responses to cytokines has been assessed through the analysis of receptor mutations which disrupt Stat activation and more recently by disruption of the genes in mice. Our results have demonstrated that the activation of Stat5a and Stat5b by erythropoietin is critical for the activation of a number of immediate early genes but is not required for a mitogenic response. Mice in which the genes for Stat4 and Stat6 are disrupted are viable but lack functions that are mediated by interleukin 12 (IL-12) or IL-4, respectively, suggesting that these Stats perform very specific functions in immune responses.