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-3R(alpha)) and a signal transducing (beta) subunit (IL-3R(beta)) 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.