Mapping of the S RBD mutants on the structure. The molecular surface diagrams of S RBD are shown as the top views in the solid and translucent models. The S RBD surface is in yellow, mutations that significantly affect the binding to ACE2 are in red and those do not affect the binding in cyan. (A) Shown are the solid surface diagrams using the structure of S RBD (left panel) and related by 180° rotations (right panel). The residues that decrease the receptor binding as observed in the experiment and exposed in the structure are labeled (R426, N473). (B) The same surface diagrams as in A but with transparency which are related by 180° rotations. The buried residues, which reduce the receptor binding as observed in the experiment, are seen as blurred red.