Interleukin 9 (IL-9) is produced by activated T helper 2 (Th2) lymphocytes. IL-9 regulates many cell types involved in classical Th2-associated asthma responses, including B and T lymphocytes, mast cells, eosinophils, and epithelial cells. The mouse IL-9 cDNA encodes a 144 amino acid precursor protein with an 18 amino acid signal peptide that is cleaved to form the mature cysteine-rich protein with a molecular mass of 14 kDa. Native mouse IL-9 is heavily glycosylated. Recently, TGF beta has been reported to play an important role in the differentiation of IL-17-producing T helper cells and inducible regulatory T cells by reprogramming Th2 cells to secrete IL-9, thereby driving the differentiation of Th9 cells directly.