von Willebrand Factor (vWF) is a glycoprotein produced by bone marrow cells and endothelial cells lining the inside surface of blood vessels. Its primary function is platelet adhesion, binding to Factor VIII, collagen and platelets, to coagulate blood at the site of wounding. The enzyme VWFCP, or vWF-cleaving protease, facilitates clotting by cutting the protein into subunits, increasing its binding capacity.

Deficiency or dysfunction of the protein increases the tendency of wounds to bleed, or to bleed more.  Over 300 gene mutations have been identified and classified into three types.  Type 1 von Willebrand Factor disease is characterized by reduced amounts in the bloodstream, Type 2 by reduced binding ability and Type 3 by a nonfunctional protein.