GM-CSF, Granulocyte-macrophage colony-stimulating factor, is a protein secreted by macrophages, T cells, mast cells, endothelial cells, and fibroblasts. By fluorescence in situ hybridization, the GM-CSF gene is mapped to 5q31.1. GM-CSF is a cytokine that functions as a white blood cell growth factor. GM-CSF stimulates stem cells to produce granulocytes (neutrophils, eosinophils, and basophils) and monocytes. GM-CSF is an essential regulator of neutrophil function.