CD105 (endoglin) is a disulfide-linked homodimeric cell membrane glycoprotein. It was initially discovered in a human pre-B-cell line. CD105 functions as a receptor for transforming growth factor (TGF) -β1 and -β3, and modulates TGF-β signaling through interactions with TGF-β receptors 1 and/or 2. CD105 is a proliferation-associated and hypoxia-inducible protein abundantly expressed in angiogenic endothelial cells. Tumor microvessel density assessed by CD105 immunohistochemical staining in paraffin-embedded tissue sections correlates significantly with tumor aggressiveness and prognosis in many types solid tumors.