Supersecondary structure topology for the doubly wound open α/β fold of the vWf-A domain. α-Helices are represented as cylinders, for which those above the central β-sheet are shaded and those below are unshaded. The β-strands are shown as arrows and labeled to follow Fig 1. N and C denote the N- and C-terminus, respectively. The positions of mutations are shown on the appropriate topology diagram as filled circles. Residues important in ristocetin binding are shown as clear circles in the vWF-A1 domain. Residues important in the protease cleavage site Y842-M843 are likewise shown as clear circles in the vWF-A2 domain.