STATs (signal transducers and activators of transcription) were originally discovered as two proteins (STAT 1 and 2) involved in IFN-a and IFN-g signal transduction. Since their initial identification, 5 more STAT proteins have been discovered (STAT 3, 4, 5a, 5b and 6). STATs undergo tyrosine phosphorylations in response to growth factor or cytokine signaling (in some cases mediated by JAK kinases [Janus Kinases 1, 2 and 3]), resulting in dimerization and translocation of STAT proteins to the nucleus. Phosphorylation at serine residues in certain STATs (STAT 1a, 3, 4 and 5) has also been reported, and appears to be required for the maximal activation of these proteins. This antibody recognizes proteins of ~91 and ~84kD, identified as STAT 1a and b. STAT 1 participates in a signaling pathway which is initiated by IFN-g.