Human Interleukin 9 (IL9) was first identified based on its proliferative activity on a human myeloid leukemic cell line. Human IL9 stimulates the proliferation of the human megakaryocytic leukemic cell line M07e. Human IL9 supports erythroid colony formation and synergizes with IL4 in the production of IgE and IgG. The interleukin 9 receptor is a member of the hematopoietin receptor superfamily. The cDNAs encoding mouse and human IL9 receptors have been isolated. The deduced mouse and human transmembrane proteins, sharing 53% amino acid sequence identity, contain 468 and 533 amino acid residues, respectively. A number of isoforms of IL9R, including a putative soluble form, have also been identified. The IL9 receptor is expressed in a variety of hematopoietic cells including T cells, neutrophils, mast cells, and macrophages.