IL-34 is secreted as a dimeric protein that is expressed in a variety of tissues, but is most abundant in the spleen. It shares the M-CSF Receptor, also known as CSF-1R and CD115, with CSF1 (also called M-CSF). IL-34 stimulates monocyte proliferation, promotes survival and proliferation of human peripheral blood monocytes, and stimulates macrophage colony formation by human bone marrow cells. It also stimulates phosphorylation of MAPK1/ERK2 and MAPK3/ERK1.