Single-chain membrane glycoprotein that forms a non-covalent complex with GPIb. (MW 23 kDa) Reactivity with resting and activated platelets, weakly on monocytes, megakaryocytes and attachment site for the platelet plasma membrane to the submembrane cytoskeleton. GPIb/IX complex, functions as the receptor for ristocetin-induced binding of von Willebrand factor and as the von Willebrand factor-depend adhesion receptor.