Signal transducer and activator of transcription 5A (STAT5) carries out a dual function: signal transduction and activation of transcription. STAT5 is tyrosine phosphorylated in response to IL-2, IL-3, IL-7, IL-15, GM-CSF, growth hormone, prolactin, erythropoietin and thrombopoietin. Phosphorylation results in translocation into the nucleus, where it binds to the GAS element and activates transcription. Tyrosine phosphorylation is required for DNA-binding activity and dimerization. Serine phosphorylation is also required for maximal transcriptional activity.