Factor VIII related antigen or von Willebrand factor is a multimeric glycoprotein, which binds to platelet glycoprotein Ib, glycoprotein IIb/IIIa, collagen and heparin. It is synthesized by endothelial cells and stored in the Weibel-Palade granules. It mediates platelet adhesion to injured vessel walls and serves as a carrier and stabilizer for coagulation factor VIII. von Willebrand factor is one of the most useful markers to identify endothelial (or megakaryocytic) lineage of neoplasms. As not all endothelial cells synthesize / store) this molecule, about 30% of tumors of vascular origin fail to stain for factor VIII related antigen, regardless of whether they are benign or malignant. Staining for factor VIII related antigen has also been used to measure angiogenesis, an indicator of tumorrecurrence.