Colony-Stimulating Factor 1 Receptor (CSF-1R), also known as CD115 or M-CSFR. CSF-1R is a transmembrane receptor tyrosine kinase that is predominantly expressed on cells of the mononuclear phagocyte lineage, including monocytes, macrophages, and their bone marrow precursors. Its activation by ligands CSF-1 or IL-34 is essential for the development, maintenance, and functional polarization of these cells. In the context of cancer, tumor-associated macrophages (TAMs), which are often polarized to a pro-tumorigenic (M2-like) state, are critically dependent on CSF-1R signaling. TAMs promote tumor growth, angiogenesis, immunosuppression, and metastasis.