Interleukin-3, or IL-3, is a pleiotropic cytokine that is primarily secreted by activated T lymphocytes and stimulates the proliferation and differentiation of hematopoietic cells. IL-3 exerts its biological effects through a receptor which consists of a ligand-specific alpha subunit (IL-3Ralpha) and a signal transducing beta subunit (IL-3Rbeta) common to the IL-3/IL-5/GM-CSF receptors. The alpha subunits are low-affinity ligand-binding proteins while the beta subunits do not themselves bind ligand, but are required for high affinity binding by the alpha subunits. The mouse IL-3 receptor has two distinct beta subunits, one that functions only in IL-3-mediated cell signaling and a second that is shared with IL-5 and GM-CSF. The murine beta subunits are 91% homologous at the amino acid level but only 56% homologous to the human beta subunit.