Macrophage colony-stimulating factor 1 receptor (CSF1R) is a member of the type III subfamily of receptor tyrosine kinases that also includes receptors for SCF and PDGF. These receptors each contain five immunoglobulin-like domains in their extracellular domain (ECD) and a split kinase domain in their intracellular region. CSF1R is expressed primarily on cells of the monocyte/macrophage lineage, dendritic cells, stem cells and in the developing placenta. CSF1 and its receptor (CSF1R, product of c-fms proto-oncogene) were initially implicated as essential for normal monocyte development as well as for trophoblastic implantation. It plays an important role in the regulation of osteoclast proliferation and differentiation, the regulation of bone resorption, and is required for normal bone and tooth development. It is required for normal male and female fertility, and for normal development of milk ducts and acinar structures in the mammary gland during pregnancy. Aberrant expression of CSF1 or CSF1R may play a role in inflammatory diseases, such as rheumatoid arthritis, glomerulonephritis, atherosclerosis, and allograft rejection.