The 9-4D2-1E4 monoclonal antibody specifically binds to CD115 which is also known as Colony stimulating factor 1 receptor (CSF-1R) or Macrophage colony-stimulating factor 1 receptor (M-CSFR). This type I transmembrane glycoprotein is a receptor tyrosine kinase (RTK) that belongs to the Ig superfamily. It is expressed on a variety of cells including those committed to the mononuclear phagocyte lineage, such as, monocytes, macrophages, and osteoclasts. CSF-1 binds to and signals through CSF-1R homodimers which undergo tyrosine autophosphorylation and transduce downstream signaling pathways resulting in cytoskeletal reorganization and gene expression. CSF-1R activation stimulates the proliferation, differentiation, and survival of cells within the mononuclear phagocyte system. Acting through CD115, CSF-1 induces macrophage spreading and motility, and in combination with RANKL, CSF-1 drives the differentiation of mononuclear phagocytes to become osteoclasts. Interleukin-34 (IL-34) is another ligand for CD115 that can induce similar, as well as, some different biological responses by CD115-positive target cells.