Signal transduction and activator of transcription 4 (STAT4) is a member of the Jak/STAT signal transduction pathway. STAT4 mediates cytokine signaling by acting as a signal transducer in the cytoplasm and, upon phosphorylation, homo or heterodimerizes with other STAT proteins, which then translocate to the nucleus and activate transcription of specific genes. STAT4 is expressed in subset of tissues including spleen, heart, brain, peripheral blood cells and testis. STAT4 is involved in mediating IL-12 response in lymphocytes and regulating T helper cell differentiation.