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.