Von Willebrand factor is an adhesive protein which is present in blood and in endothelium. It is responsible for adhesion of platelets to damaged vascular endothelium, among other things. Von Willebrand factor is made up of several domains. The A domain (containing several repeats) is responsible for adhesion. Von Willebrand factor A1 domain (which binds to platelet glycoprotein Ib) has been crystallized and is shown below. The A3 domain (not shown) binds to collagens. Von Willebrand Factor, A1 Domain (PDB file 1AUQ)