Protein structure with respect to their geometrical region. (a) Schematic diagram representing the interior, interface, and surface of longitudinal section of a protein domain. (b) An example of a 3D structure (SCOP id: d1a25a_) which corresponds to a schematic diagram (a). It shows the three areas of a domain (red: protein surface, blue: protein interior, filled-in space model: interaction interface). Interface regions are represented as a space-fill model to distinguish them from other regions.