Carries out a dual function: signal transduction and activation of transcription. Forms a homodimer or a heterodimer with a related family member (By similarity). Interacts with ARL2BP. Translocated into the nucleus in response to phosphorylation. Tyrosine phosphorylated. Serine phosphorylation is also required for maximal transcriptional activity (By similarity). Belongs to the transcription factor STAT family.