Macrophage Colony Stimulating Factor (M-CSF), also known as CSF-1, can be produced by a number of cells, including fibroblasts, monocytes, activated macrophages, secretory epithelial cells of the endometrium, endothelial cells activated by LPS or cytokines, and bone marrow stromal cells. In addition to its namesake activity, M-CSF can stimulate the proliferation of isolated macrophages, augment the production and release of cytokines and other inflammatory modulators from macrophages, enhance macrophage antibody-dependent cell-mediated cytotoxicity, prime and enhance macrophages in their ability to kill tumor cells and other microorganisms, stimulate pinocytosis, and support osteoclast differentiation. M-CSF binds and activates a 165 kDa glycoprotein of the receptor tyrosine kinase subclass III or the RTK subfamily. Due to alternative splicing of a single M-CSF gene and due to variations in glycosylation, the molecular weight of natural soluble M-CSF ranges from 44-86 kDa. Human and mouse M-CSF share approximately 80% sequence homology in the conserved N-terminal region of 150 amino acids. Although human M-CSF is active in murine systems, mouse M-CSF appears to be species-specific in its actions.