Granulocyte macrophage colony stimulating factor receptor alpha (GM-CSF R alpha ), also known as CD116, is a component of the receptor complex that mediates cellular responses to GM-CSF. GM-CSF promotes the differentiation and mobilization of granulocyte-macrophage, erythroid, megakaryocyte, and eosinophil progenitors. It enhances the activation of myeloid cell effector functions and plays a role in the development of Th1 biased immune responses, allergic inflammation, and autoimmunity (1-4). Mature mouse GM-CSF R alpha is an 80 kDa type I transmembrane glycoprotein that consists of a 298 amino acid (aa) extracellular domain (ECD) with two fibronectin type III domains and a juxtamembrane WSXWS motif, a 21 aa transmembrane segment, and a 40 aa cytoplasmic domain (5). Within the ECD, mouse GM-CSF R alpha shares approximately 33% and 58% aa sequence identity with human and rat GM-CSF R alpha, respectively. Soluble forms of the human receptor retain the ability to bind GM-CSF (6, 7). GM-CSF R alpha is expressed on hematopoietic stem cells, progenitor and differentiated cells in the myeloid lineage, vascular endothelial cells, placenta, and non‑hematopoietic solid tumor cells (8). GM-CSF R alpha associates with the common beta chain/CD131 ( beta _c), a 135 kDa transmembrane protein that is also the signal transducing component of the receptors for IL-3 and IL-5 (9, 10). Association with beta _c converts GM-CSF R alpha from a low affinity to a high affinity receptor for GM-CSF (9-11). The shared usage of beta _c underlies the synergism between GM-CSF, IL-3, and IL-5 in their effects on myeloid cell differentiation and activation (1, 2).