STATs (signal transducers and activators of transcription) are a family of cytoplasmic latent transcription factors that are activated to regulate gene expression in response to a large number of extracellular signaling polypeptides including cytokines, interferons, and growth factors. After phosphorylation by JAK tyrosine kinases, STATs enter the nucleus to regulate transcription of many different genes. Among the seven STATs (Stat1, Stat2, Stat3, Stat4, Stat5a, Stat5b, and Stat6), Stat1, Stat3, Stat5alpha, and Stat5beta have a wide activation profile. STAT1 is activated by many different ligands including IFN family (IFN-alpha, IFN-beta, IFN-gamma and IL-10), gp130 family (IL-6, IL-11, LIF, CNTF, and G-CSF), and receptor tyrosine kinases (EGF, PDGF, and CSF-1). STAT1 has two forms, the 91 kDa STAT1alpha and the 84 kDa STAT1beta which are encoded by the same gene with splicing variant.