Colony-stimulating factors (CSFs) are secreted glycoproteins which bind to receptor proteins on the surfaces of hemopoietic stem cells and thereby activate intracellular signaling pathways which can cause the cells to proliferate and differentiate into a specific kind of blood cell. The substance which was found to stimulate formation of colonies of macrophages, for instance, was called macrophage colony-stimulating factor, and so on.

Macrophage Colony-stimulating factor is a secreted cytokine which influences hemopoietic stem cells to differentiate into macrophages or other related cell types.The active form of the protein is found extracellularly as a disulfide-linked homodimer, and is thought to be produced by proteolytic cleavage of membrane-bound precursors.