(a) Ribbon representation of the three-dimensional structure of apo BACE1. The N- and C-terminal lobes are coloured cyan and yellow, respectively. The Asp-dyad residues, Asp32 and Asp228, are displayed as sticks. Residues 67–78 (including the flap) are coloured red and the flap is shown in its `open' conformation. (b) Ribbon and molecular-surface representations of the three-dimensional structure of the BACE1 `dimer' generated based on PDB entry 1w50. The monomer is displayed in space-filling format. The symmetry-related monomer is displayed in ribbon format. A close-up of two the two flaps in the `dimer' is displayed in the top left corner.