Recognizes a multimeric glycoprotein of 270kDa, identified as factor VIII related antigen or von Willebrand factor. This protein has functional binding domains to platelet glycoprotein Ib, glycoprotein IIb/IIIa, collagen and heparin. von Willebrand factor 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.This antibody reacts specifically with the endothelial cells of normal, reactive, and neoplastic blood and lymphatic vessels and shows a finely granular cytoplasmic staining. It also reacts with endocardium, platelets, and megakaryocytes. von Willebrand factor is one of the most useful markers to identify endothelial (or megakaryocytic) lineage of neoplasms. However, because not all endothelial cells synthesize (or 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 which reportedly predicts tumor recurrence.