The Macromolecular Crystallographic Information File (mmCIF) also known as PDBx/mmCIF is a standard text file format for representing macromolecular structure data, developed by the International Union of Crystallography (IUCr) and the Protein Data Bank It is an extension of the Crystallographic Information File (CIF), specifically for macromolecular data, such as proteins and nucleic acids, incorporating elements from the PDB file format. 
mmCIF is intended as an alternative to the Protein Data Bank (PDB) format and is now the default format used by the Protein Data Bank.
mmCIF was designed to address limitations of the PDB format in terms of capacity and flexibility, especially with the increasing size and complexity of macromolecular structures being determined. 
The format is part of the larger Crystallographic Information Framework, a system of exchange protocols based on data dictionaries and relational rules expressible in different machine-readable manifestations, including, but not restricted to, the original Crystallographic Information File and XML.