STATs (signal transducers and activators of transcription) were originally discovered as two proteins (STAT1 and STAT2) which were involved in interferon alpha (IFN alpha) and IFN gamma signal transduction. Since then, several additional STAT proteins have been identified (STAT3, 4, 5a, 5b, and 6). STATs undergo tyrosine phosphorylation in response to growth factor or cytokine signaling. This phosphorylation results in dimerization and translocation of STAT proteins to the nucleus. In some cases this process is mediated by JAK Kinases (Janus Kinases 1, 2, and 3). For maximum activation of these proteins, phosphorylation at specific tyrosine and serine residues may be required in STAT1 alpha, 3, 4, and 5. Specific functions of the STAT proteins are poorly defined at this time. They are thought to be involved in a variety of systems including antiviral responses and cell transformation. STAT1 deficient knock-out mice are unable to induce IFN regulated genes and are extremely susceptible to viral diseases. After phosphorylation, STAT1 and STAT2 form heterodimers that function as more potent inducers of transcription than the STAT1 homodimer.