Domain definition categories. Block-diagram domain-architecture schematics representing strategy for domain definition categories are shown on the left with the corresponding structures (1amu for a and b; 1qme for c and d) on the right. A schematic sequence-view representing the position of domains in the polypeptide chain is shown below each block-diagram. Residue numbers are marked at linkers joining domains, with N and C marking the termini. Only a part of the protein structure and corresponding schematics are shown for clarity. Broken lines indicate omitted domains from the structures. Terminal extensions that protrude from one domain yet interact with another domain are defined (a) by sequence proximity or (b) by structure proximity. Protruding domain insertions that interact with neighboring domains are defined (c) by sequence proximity or by (d) structural proximity resulting in a composite domain.