Endoglin (ENG, CD105) is a homodimeric cell membrane glycoprotein of 180 kDa, composed of disulphide-linked subunits of 90-95 kDa. Endoglin is a proliferation-associated and hypoxia-inducible protein mainly expressed on vascular endothelial cells. It acts as an accessory receptor for transforming growth factor beta (TFG-β) and is involved in vascular development and remodelling. The important role of Endoglin in angiogenesis and in tumor progression makes it an ideal target for antiangiogenic therapy and a good marker for tumor prognosis. The extracellular domain of membrane-bound Endoglin can be proteolytically cleaved, releasing a soluble form of Endoglin (sCD105). Increased levels of sCD105 are linked to the pathogenesis of severe vascular disease, and also correlate with poor prognosis in patients suffering from various types of cancer.