IL-34 is a glycoprotein that plays important roles in the proliferation, survival, and differentiation of monocytes and macrophages. IL-34 is expressed in the heart, brain, lung, liver, kidney, thymus, spleen, and other organs. It specifically binds to CD115/M-CSF receptor (CSF-1 receptor or c-Fms) and activates ERK1/2 phosphorylation. The function of IL-34 overlaps with CSF-1 to regulate myeloid lineage development. IL-34 also promotes inflammatory processes and regulates osteoclast proliferation, differentiation, and bone reabsorption.