Granulocyte-macrophage colony-stimulating factor (GM-CSF), also known as colony-stimulating factor 2 (CSF2), is a monomeric glycoprotein secreted by macrophages, T cells, mast cells, natural killer cells, endothelial cells and fibroblasts that functions as a cytokine. Unlike granulocyte colony-stimulating factor, which specifically promotes neutrophil proliferation and maturation, GM-CSF affects more cell types, especially macrophages and eosinophils. GM-CSF is a monomeric glycoprotein that functions as a cytokine—it is a white blood cell growth factor. GM-CSF also has some effects on mature cells of the immune system. These include, for example, enhancing neutrophil migration and causing an alteration of the receptors expressed on the cells surface. GM-CSF signals via signal transducer and activator of transcription, STAT5. In macrophages, it has also been shown to signal via STAT3. The cytokine activates macrophages to inhibit fungal survival. It induces deprivation in intracellular free zinc and increases production of reactive oxygen species that culminate in fungal zinc starvation and toxicity. Thus, GM-CSF facilitates development of the immune system and promotes defense against infections. GM-CSF also plays a role in embryonic development by functioning as an embryokine produced by reproductive tract. GM-CSF is found in high levels in joints with rheumatoid arthritis and blocking GM-CSF as a biological target may reduce the inflammation or damage. In critically ill patients GM-CSF has been trialled as a therapy for the immunosuppression of critical illness, and has shown promise restoring monocyte and neutrophil function, although the impact on patient outcomes is currently unclear and awaits larger studies.