The AFS98 monoclonal antibody specifically recognizes CD115 which is also known as Colony stimulating factor 1 Receptor (CSF-1R), Macrophage colony-stimulating factor 1 receptor (M-CSFR), or c-fms. This type I transmembrane glycoprotein is a receptor tyrosine kinase (RTK) that is encoded by Csf1r which belongs to the Ig superfamily. CD115 (CSF-1R) is comprised of an extracellular ligand-binding domain that is followed by a single transmembrane segment and a split intracellular tyrosine kinase domain. This receptor is expressed on monocytes, macrophages, dendritic cells, osteoclasts, and their precursors. Colony-stimulating factor 1 (CSF-1), also known as Macrophage colony-stimulating factor (M-CSF), binds to and signals through CD115 (CSF-1R) homodimers which undergo tyrosine autophosphorylation. The activated receptors transduce intracellular signals resulting in cytoskeletal reorganization and gene expression involved in the proliferation, differentiation, and survival of CSF-1-responding cells. Through CD115 (CSF-1R), CSF-1 regulates the release of proinflammatory cytokines and other mediators from macrophages and plays a role in the bone resorption activity of osteoclasts. Interleukin-34 (IL-34) is another ligand for CD115 (CSF-1R) that can induce similar, as well as, some different biological responses by target cells that express this receptor.