STAT 3 is an 88 kD member of the STAT (signal transducer and activators of transcription) protein family that is phosphorylated in response to cytokine receptor-associated kinase activity. Phosphorylation of STAT 3 induces nuclear translocation to activate transcription. STAT3 is expressed in brain, heart, placenta, lung, liver, skeletal muscle, kidney, and pancreas. STAT 3 forms both homo- and heterotrimers and is involved in the activation of genes required for cell growth and apoptosis. STAT 3 is also involved in gp130 signaling and binds to IL-6 response elements in various acute phase protein promoters. STAT 3 is phosphorylated by signaling from IFNs, EGF, FGF, IL-5, HGF, LIF, and BMP2. STAT 3 activity is inhibited by PIAS3 and GRIM-19 and can also be regulated by Rac1 protein. STAT 3 interacts with Rac1, STAT 1, and NCOA1 proteins. The Poly6246 antibody recognizes the C-terminal region of human, mouse, and rat STAT3 and has been shown to be useful for Western blotting, immunoprecipitation, and immunohistochemistry. The Poly6246 antibody does not react with STAT3b.