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 (By similarity). Interacts with NCOA1 via its C-terminal LXXLL motif. Translocated into the nucleus in response to phosphorylation. Tyrosine phosphorylated on Tyr-641 following stimulation by IL4/interleukin-4 (PubMed:27796300). Tyrosine phosphorylated following stimulation by IL3/interleukin-3 (By similarity). Dephosphorylation on tyrosine residues by PTPN2 negatively regulates the IL4/interleukin-4 mediated signaling (PubMed:17210636). Mono-ADP-ribosylated by PARP14. Belongs to the transcription factor STAT family.