Carries out a dual function: signal transduction and activation of transcription. Mediates cellular responses to the cytokine KITLG/SCF and other growth factors. Mediates cellular responses to ERBB4. May mediate cellular responses to activated FGFR1, FGFR2, FGFR3 and FGFR4. Binds to the GAS element and activates PRL-induced transcription. Regulates the expression of milk proteins during lactation. Forms a homodimer or a heterodimer with a related family member. Binds NR3C1 (By similarity). Interacts with NCOA1 and SOCS7. Interacts with ERBB4. Translocated into the nucleus in response to phosphorylation. Tyrosine phosphorylated in response to KITLG/SCF, IL2, IL3, IL7, IL15, CSF2/GMCSF, GH1, PRL, EPO and THPO (By similarity). Activated KIT promotes phosphorylation on tyrosine residues and subsequent translocation to the nucleus (PubMed:21135090). Tyrosine phosphorylated in response to constitutively activated FGFR1, FGFR2, FGFR3 and FGFR4 (By similarity). Tyrosine phosphorylation is required for DNA-binding activity and dimerization. Serine phosphorylation is also required for maximal transcriptional activity (By similarity). Tyrosine phosphorylated in response to signaling via activated FLT3; wild-type FLT3 results in much weaker phosphorylation than constitutively activated mutant FLT3 (PubMed:14504097). Alternatively, can be phosphorylated by JAK2 at Tyr-694 (PubMed:12529425). ISGylated. Belongs to the transcription factor STAT family. It was reported that dephosphorylation on tyrosine residues by PTPN2 would negatively regulate prolactin signaling pathway (PubMed:11773439). However, the corresponding article has been retracted (PubMed:24319783). STAT5 entry