Signal transducer and activator of transcription (STAT), named after their dual role, are latent cytoplasmic transcription factors that mediate various biological responses. The activation of STAT proteins is largely mediated by phosphorylation of C-terminal transactivation domain through Janus kinases (JAKs) and mitogen-activated protein kinases (MAPKs), which allows the activated STATs to dimerize and to translocate into the nucleus. By modulating target gene expression, STAT proteins play an important role in mediating a broad range of biological processes such as cell proliferation, survival, apoptosis, and differentiation. Seven mammalian members of the STAT family are known (STAT1, 2, 3, 4, 5a, 5b, and 6) and they all share common features and structure. STAT3 is activated by growth factors and oncogenic kinases where it mediates transcriptional activation of genes encoding apoptosis inhibitors, cell-cycle regulators and inducers of angiogenesis.