Interleukin-34 (IL-34) is a 30 kDa cytokine with 223 amino acid residues. IL-34 is a ligand of CSF-1R and secreted from CSF-1R expressing cells like myeloid cells, epithelial cells, endothelial cells, fibroblasts, and neurons. IL-34 mediates cell survival, proliferation, cytokine/chemokine expression, and migration via activating several signaling pathways. Moreover, it also plays a critical role in developing and maintaining the mononuclear phagocytic system.