CD105 is also known as Endoglin. It is a type I integral membrane homodimer protein with subunits of 90 kD found on vascular endothelial cells and syncytiotrophoblasts of placenta. CD105 is weakly expressed on stromal fibroblasts. It is also expressed on activated monocytes and tissue macrophages. Expression of CD105 is increased on activated endothelium in tissues undergoing angiogenesis, such as in tumors, or in cases of wound healing or dermal inflammation. CD105 is a component of the TGF-beta receptor system in human umbilical vein endothelial cells and binds TGF-beta1 and beta3 with high affinity but does not bind to TGF-beta2.