IL-9 is an immunoregulatory cytokine produced by IL-2 activated Th2 lymphocytes. IL-9 enhances the proliferation of T lymphocytes, mast cells, erthroid precursor cells and megakaryoblastic leukemia cell lines. Over-expression of IL-9 has been implicated in the pathogenesis of anaplastic lymphoma and Hodgkin's disease. Whereas murine IL-9 can function on human cells, human IL-9 is inactive on mouse cells. Recombinant human IL-9 is a 14.0 kDa protein of 127 amino acid residues including 10 cysteine residues which are fully conserved between the human murine proteins.