Different definitions of residues' structural environment or neighbourhood. (A) Single residue (red), i.e. no environment. (B) 9-residue sliding window (as in Sikic et al. ); central residue is shown in red and flanking residues in yellow. (C) Euclidean distance cut-off; residues enclosed in a sphere of radius R = 15 Angstroms (yellow) as in Porollo et al. , centred on the given residue (red). (D) Voronoi Diagrams; residue of interest (red) with colour gradient showing neighbouring residues; orange: residues sharing more than 16 edges with residue of interest; yellow: between 8 to 16; green: less than 8. Inset shows the 2D projection of a VD between two residues.