GM-CSF is a hematopoietic growth factor secreted by macrophages, T cells, mast cells, natural killer cells, endothelial cells and fibroblasts that functions as a cytokine. GM-CSF stimulates the development of neutrophils and macrophages, and promotes the proliferation and development of early erythroid megakaryocytic and eosinophilic progenitor cells. GM-CSF binds to receptor heterodimers consisting of a GM-CSF-specific alpha chain and the common beta chain that is shared by the high-affinity receptors for IL-3 and GM-CSF.