Signal Transducer and Activator of Transcription 6 (STAT6) is a member of the Jak/STAT signal transduction pathway and mediates cytokine signaling by IL-4. IL-4 activation leads to tyrosine phosphorylation and accumulation in the nucleus of STAT6 homo and heterodimers. STAT6 acts as a signal transducer in the cytoplasm and, upon phosphorylation at Y641, translocates to the nucleus and binds to the DNA consensus site TTCN ₄GAA. Knockout studies in mice suggest that STAT6 functions in differentiation of T helper 2 (Th2) cells, expression of cell surface markers, and class switch of immunoglobulins. STAT6 transcriptional activation occurs by binding p100 and p160 class (NcoA1, Src-1) co-activators. Gene expression studies indicate that STAT6 mediates cell proliferation and anti-apoptotic effects of IL-4.