The mmCIF (Macromolecular Crystallographic Information File) format is a standard text-based file format used extensively in structural biology for representing and exchanging macromolecular structure data. It is an extension of the more general CIF (Crystallographic Information File) standard, specifically tailored for biological macromolecules like proteins, nucleic acids, and their complexes determined by techniques such as X-ray crystallography, NMR spectroscopy, and cryo-electron microscopy.
Unlike older, fixed-column formats like the original PDB format, mmCIF is a self-describing, hierarchical, and flexible format. Data items are identified by unique names (tags) preceded by an underscore (e.g., _atom_site.label_atom_id), making the file human-readable and machine-parsable without requiring external dictionaries. The format supports complex data structures, including tables (loops), and allows for the inclusion of extensive metadata beyond just atomic coordinates, such as experimental details, refinement statistics, ligand information, and biological assembly details.
The mmCIF format was developed by the International Union of Crystallography (IUCr) and is the primary format used by the worldwide Protein Data Bank (wwPDB) for archiving and distributing structural data. Its adoption ensures data integrity, facilitates automated processing, and promotes interoperability among various structural biology software tools. It addresses many limitations of the legacy PDB format, providing a more robust and extensible framework for the ever-growing complexity of macromolecular structures and associated experimental data. Its flexibility allows it to evolve with new experimental techniques and data types.