Transcription factor that binds to target promoter sequences and activates transcription upon il6st/gp130 stimulation. Mediates ventralization of embryos, at least in part via inhibition of smad2 signaling. Required for hairy2 to induce dll1/delta1 and promote neural crest cell proliferation and differentiation. Involved in TGFbeta-mediated mesoderm induction in early embryos, acting downstream of map3k7/tak1 and nlk.2 (By similarity). Forms a homodimer or a heterodimer with a related family member. Interacts with nlk.2. Shuttles between the nucleus and the cytoplasm. Constitutive nuclear presence is independent of tyrosine phosphorylation (By similarity). Phosphorylation of both tyrosine and serine residues, together with dimerization, is required for mesoderm induction. Involved in the gp130-mediated signaling pathway. Belongs to the transcription factor STAT family.