GM-CSF plays a key role in signaling emergency hematopoiesis (predominantly myelopoiesis) in response to infection. This includes the development of granulocytes and macrophages in the bone marrow and their maintenance, survival, and functional activation at sites of injury or insult. The receptor for GM-CSF is a heterodimer that comprises a major binding subunit (GMRα) and a major signaling subunit (βc). The receptor subunits are always coexpressed on the surface of leukocytes, with βc being expressed at lower levels than GMRα. Certain nonhematopoietic cell types have also been reported to express the GM-CSF receptor and to respond to GM-CSF stimulation in vitro, although the in vivo significance of these observations remains uncertain.