STATs (signal transducers and activators of transcription) are a family of cytoplamic latent transcription factors that are activated to regulate gene expression in response to a large number of extracellular signalling peptides 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, Stat5a and Stat5b have wide activation profiles. Stat1 is activated by many different ligands including IFN family (IFN-a, IFN-b, IFN-r, and IL-10), gp130 family (IL-6, IL-11, LIF, CNTF, and G-CSF-1), and receptor tyrosine kinases (EGF, PDGF, and CSF-1). Stat1 has two forms, the 91 kDa STAT1α and the 84 kDa STAT1β, which are coded by the same gene with splicing variant.