Carries out a dual function: signal transduction and activation of transcription. Mediates cellular responses to the cytokine KITLG/SCF and other growth factors. 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. Interacts with NCOA1 and SOCS7 (By similarity). Binds NR3C1. Interacts with ERBB4. Translocated into the nucleus in response to phosphorylation. In the virgin, found in most tissues except brain and muscle. During lactation, abundantly found in mammary tissue, as well as in other secretory organs such as salivary gland and seminal vesicle. ISGylated. Tyrosine phosphorylated in response to KITLG/SCF, IL2, IL3, IL7, IL15, CSF2/GMCSF, GH1, PRL, EPO and THPO (PubMed:16837552). 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 (PubMed:7720707). 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:11090077, PubMed:21262971). Alternatively, can be phosphorylated by JAK2 at Tyr-694 (By similarity). Belongs to the transcription factor STAT family.