A schematic model for the membrane-proximal portion of the GPIb-IX complex. (A) Cartoon diagrams related by a 90-degree rotation showing the GPIbα chain TM domain (in green) extending toward the N-terminus (top), GPIbβ ectodomain and TM domain (in sky blue and dark blue), and GPIX ectodomain and TM domain (in purple). Tyr106 from GPIbβ₁ is shown as stick along with interchain disulphides. (B) Same view as panel A, except without TM domains. GPIbβ is colored white, showing residues affected by BSS and human platelet-specific alloantigen (HPA) mutations. Side chains of GPIbβ residues Ala108 (blue), Pro74 (orange), and Gly15 (red) are shown in space-filling mode; Tyr106 is shown as stick (gray).