Platelet membrane glycoprotein Ib (GPIba) is comprised of an a and b subunit linked by disulfide bonds. GPIba (also known as CD42b) is a 135 kDa membrane protein subunit that binds a variety of adhesive and procoagulant ligands, including von Willebrand factor. Cleavage of GPIba by the “sheddase” ADAM17 releases the ectodomain glycocalicin into plasma. ADAM17 cleaves GPIba at Gly464-Val465.