Signal transducer and activator of transcription 5Bis a protein in humans that is encoded by STAT5B gene. Signal transducer and activator of transcription 5B is a protein in humans that is encoded by STAT5B gene. It carries out a dual function: signal transduction and activation of transcription. Mediates cellular responses to the cytokine KITLG/SCF and other growth factors. Binds to the GAS element and activates PRL-induced transcription. Positively regulates hematopoietic/erythroid differentiation.