Purity >95% as determined by SDS-PAGE. Interleukin 9 (IL-9) is a cytokine produced by type 2 T helper (Th2) cells and regulates hematopoietic cells. IL-9 signals through the interleukin 9 receptor (IL9R) to activate STAT signaling. IL-9 functions to induce cell proliferation, prevent cell apoptosis, and is associated with asthma and airway hyperresponsiveness.