Cysteine residues in GP Ibα and GP Ibβ. The disulfide bonds in each subunit are marked by connected lines. The transmembrane domains are marked by filled boxes. C65 in GP Ibα is buried in the N-terminal domain and is unpaired.^17,18  GP Ibα has 2 membrane-proximal Cys residues (C484 and C485), whereas GP Ibβ has one (C122). C122 forms a disulfide bond with a Cys residue in GP Ibα, the identity of which remains unclear.