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Platelet glycoprotein Ib alpha chain also known as glycoprotein Ib （platelet）, alpha polypeptide or CD42b （Cluster of Differentiation 42b）, is a protein that in humans is encoded by the GP1BA gene. Glycoprotein Ib （GPIb） is a platelet surface membrane glycoprotein composed of a heterodimer, an alpha chain and a beta chain, that are linked by disulfide bonds. The GpIb functions as a receptor for von Willebrand factor （VWF）. The complete receptor complex includes noncovalent association of the alpha and beta subunits with platelet glycoprotein IX and platelet glycoprotein V to form the glycoprotein Ib-IX-V complex. The binding of the GP Ib-IX-V complex to VWF facilitates initial platelet adhesion to vascular subendothelium after vascular injury, and also initiates signaling events within the platelet that lead to enhanced platelet activation, thrombosis, and hemostasis.