IL3 (interleukin 3), also known as IL-3, is a potent growth-promoting cytokine that belongs to the IL-3 family. Interleukin-3 is a cytokine that regulates the proliferation, differentiation, activation, and survival of myeloid progenitor cells and the mast cells, eosinophils, basophils, neutrophils, monocytes, megakaryocytes, and erythroid cells that they give rise to (1–3). Although IL-3 was first defined as a CSF (multi-CSF), it is not essential to hemopoiesis, and it may function in vivo pre dominantly as a proinflammatory cytokine activating mature myeloid cells. The IL-3 gene exists within a one megabase conserved cytokine gene cluster that also contains the genes for IL-4, IL-5, IL-13, and GM-CSF. IL3/IL-3 has been shown to also possess neurotrophic activity, and it may be associated with neurologic disorders.