Example of surface subregions and the degree of packing density. The interface of the thymidylate synthase (PDB code: 2TSC). (A) One of the two identical subunits interacts with its partner (translucently green region) through the interface. The central interface region is colored red, the peripheral interface region is colored red, and the non-interface region is colored gray. (B) The colors of interface residues are ramped from blue (large WCN, i.e., high packing density, more rigid) to red (small WCN, i.e., small packing density, more flexible), and the non-interface residues are shown in gray.