IL-34 is a homodimeric glycoprotein which contains 241 amino acids. IL-34 is expressed in 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 differentiation, proliferation, and survival. Elevated levels of IL-34 are seen in the plasma of rheumatoid arthritis patients, but can be decreased in response to therapy. IL-34 levels are also increased in the serum of cancer patients and associated with poor prognosis. Mouse IL-34 shares 71% sequence identity with human IL-34 on the amino acid level.