Catalytic domain of the Protein Tyrosine Kinase, Colony-Stimulating Factor-1 ReceptorPTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. CSF-1R, also called c-Fms, is a member of the Platelet Derived Growth Factor Receptor (PDGFR) subfamily of proteins, which are receptor PTKs (RTKs) containing an extracellular ligand-binding region with five immunoglobulin-like domains, a transmembrane segment, and an intracellular catalytic domain. The binding of CSF-1R to its ligand, CSF-1, leads to receptor dimerization, trans phosphorylation and activation, and intracellular signaling. CSF-1R signaling is critical in the regulation of macrophages and osteoclasts. It leads to increases in gene transcription and protein translation, and induces cytoskeletal remodeling. CSF-1R signaling leads to a variety of cellular responses including survival, proliferation, and differentiation of target cells. It plays an important role in innate immunity, tissue development and function, and the pathogenesis of some diseases including atherosclerosis and cancer. CSF-1R signaling is also implicated in mammary gland development during pregnancy and lactation. Aberrant CSF-1/CSF-1R expression correlates with tumor cell invasiveness, poor clinical prognosis, and bone metastasis in breast cancer. Although the structure of the human CSF-1R catalytic domain is known, it is excluded from this specific alignment model because it contains a deletion in its sequence. The CSF-1R subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.