CD105 or Endoglin is a Type I transmembrane protein, which is highly expressed on vascular endothelial cells. It exists on an O- and N-glycosylated homodimer. Up regulation of endoglin expression has been demonstrated in tumor vasculature and proliferating cells, suggesting that it is a proliferation associated endothelial marker. CD105 binds to TGF-ß1 and ß3 with high affinity but not to TGF-ß2.