Interleukin 9 (IL-9) is cytokine produced by a subgroup of helper T cells, known as Th2 cells. IL-9 signals through its receptor IL-9R to induce cell proliferation and survival. Genetically, IL-9 has been identified as a gene associated with asthma. It is also thought to be a regulator of hematopoiesis. It has been shown to enhance the growth of human mast cells and megakaryoblastic leukemic cells as well as murine helper t-cell clones.