Interleukin 3 (IL-3) is a hematopoietic cytokine related to GM-CSF and IL-5. The IL-3 receptor is a heterodimer composed of an IL-3 specific but low affinity α-chain, and non-specific but high affinity β-chain. The β-chain is common to the IL-3, IL-5 and GM-CSF receptors. IL-3 is a potent growth promoting cytokine, primarily produced by Type 2 helper T cells, that regulates hematopoietic cells and leukocytes. IL-3 is clinically important in protective immunity and is also implicated in a variety of inflammatory disorders, notably allergic inflammation. IL-3 receptor is highly expressed on the surface of various cancer cells (e.g., leukemia stem cells), and it is associated with the initiation and development of acute myeloid leukemia and acute lymphoblastic leukemia.