CD109 is a a novel member of the alpha2-macroglobulin (alpha2M)/complement superfamily. It is a glycosylphosphatidylinositol (GPI)-linked cell surface glycoprotein of approximately 170 kd found on
endothelial cells, activated platelets and T-cells and on a subset of hematopoietic stem and progenitor cells. Although it has been suggested that T-cell CD109 may play a role in antibody-inducing T-helper function and it is known that platelet CD109 carries the Gov alloantigen system, the role of CD109 in hematopoietic cells remains largely unknown.
CD109 has been identified as part of the TGF-beta receptor system in human keratinocytes and upregulation of CD109 expression has been observed in several different types of tumour.