Carries out a dual function: signal transduction and activation of transcription. Involved in IL4/interleukin-4- and IL3/interleukin-3-mediated signaling. Forms a homodimer or a heterodimer with a related family member. Interacts with NCOA1 via its C-terminal LXXLL motif (By similarity). Translocated into the nucleus in response to phosphorylation. Tyrosine phosphorylated on Tyr-641 following stimulation by IL4/interleukin-4. Tyrosine phosphorylated following stimulation by IL3/interleukin-3. Dephosphorylation on tyrosine residues by PTPN2 negatively regulates the IL4/interleukin-4 mediated signaling. Mono-ADP-ribosylated by PARP14. Belongs to the transcription factor STAT family.