The von Willebrand Factor type A domain is the prototype for a protein superfamily. It possesses no significant sequence similarity to any known protein structure. Secondary structure predictions indicate a largely alternating pattern of six α-helices and six β-strands. A protein fold for this domain is proposed to correspond to a doubly-wound open twisted β-sheet structure flanked by α-helices. Close agreement was found with the GTP-binding domain of human ras-p21, provided that an extra α-helix was inserted. The structure of the predicted fold showed high compatibility with the proximate location of two Mg2+-binding Asp residues, two disulphide-bridged Cys residues, and other known functional attributes of this domain. © 1995.