Protein Data Bank archive (PDB) is the single worldwide archival repository of information about the 3D structures of proteins, nucleic acids, and complex assemblies, managed by the Worldwide PDB (wwPDB). The PDB Exchange Dictionary (PDBx) is used by the wwPDB to define data content for deposition, annotation and archiving of PDB entries. PDBx incorporates the community standard metadata representation, the Macromolecular Crystallographic Information Framework (mmCIF), orginally developed under the auspices of the International Union of Crystallography (IUCr). PDBx has been extended by the wwPDB to include descriptions of other experimental methods that produce 3D macromolecular structure models such as Nuclear Magnetic Resonance Spectroscopy, 3D Electron Microscopy and Tomography.