Signal transducer and transcription activator that mediates cellular responses to interleukins, KITLG/SCF, LEP and other growth factors.Once activated, recruits coactivators, such as NCOA1 or MED1, to the promoter region of the target gene.May mediate cellular responses to activated FGFR1, FGFR2, FGFR3 and FGFR4.