CSF-1R, also known as CD115 and M-CSFR, is a single-pass type I membrane protein and member of the platelet-derived growth factor receptor family. Structural studies of CD115 have described an Ig-like extracellular domain, a transmembrane domain, an intracellular juxtamembrane domain, a split tyrosine kinase domain, and a C-terminal tail receptor. Receptor activation induces homo-dimerization in addition to phosphorylation and ubiquitinylation of intracellular residues. The natural ligands of CD115 include M-CSF and IL-34. CD115 directly influences tissue macrophage and osteoclast differentiation and proliferation. It is expressed on monocytes/macrophages, plasmacytoid and conventional dendritic cells, and osteoclasts.