Platelet glycoprotein IX (GP9,also known as CD42a) is a relatively small surface glycoprotein present on platelets and megakaryocytes. GP9 forms a 1:1 non-covalent complex with GPIb (CD42b). The GPIb-V-IX complex functions as the vWF receptor and mediates vWF-dependent platelet adhesion to blood vessels. Defects in GP9 are a cause of Bernard-Soulier syndrome,also known as giant platelet disease.