CD109 is a glycosylphosphatidylinositol (GPI)-linked glycoprotein that belongs to the alpha2-macroglobulin family of thioester containing proteins. CD109 is associated with TGF-beta receptor I (TbRI) and inhibits TGF-beta signaling. Cleavage of CD109 at its Furin cleavage site results in the release of its large amino-terminal domain, which then binds to the TGF-beta receptor I to inhibit TGF-beta signaling. CD109 is expressed on a subset of CD34+ bone marrow cells and mesenchymal stem cells, activated platelets, activated T cells, endothelial cells, and a wide variety of tumors. Elevated CD109 expression has been considered a diagnostic/prognostic marker for several types of cancers.
