The Protein Data Bank (PDB) is a database for the three-dimensional structural data of large biological molecules, such as proteins and nucleic acids. It is a repository of atomic coordinates and other information describing proteins and other important biological macromolecules. The PDB format is a textual file format describing the three-dimensional structures of molecules held in the Protein Data Bank, now succeeded by the mmCIF format). The PDB format provides for description and annotation of protein and nucleic acid structures including atomic coordinates, secondary structure assignments, as well as atomic connectivity). The PDB is a key resource in areas of structural biology, such as structural genomics, and most major scientific journals and some funding agencies now require scientists to submit their structure data to the PDB. Many other databases use protein structures deposited in the PDB. The PDB was established in 1971 as an archive for biological macromolecular crystal structures. Its fixed-column width format is limited to 80 columns, which was based on the width of the computer punch cards that were previously used to exchange the coordinates). The PDB format has undergone many changes and revisions over the years, and as of 13 July 2011, the most recent revision is 3.30).