Helen M. Berman responds:

In order to integrate the growing number of unpublished structure entries in the PDB with the larger body of scientific literature, the PDB will assign a document object identifier (DOI) to each PDB entry.

The PDB has recently contracted with the CrossRef organization to obtain DOIs for all of the entries in the current archive and for future entries. CrossRef provides a service to the community of scholarly publishers linking DOIs to their corresponding electronic publications. The DOI has therefore become a universal electronic identifier for scientific literature. With a DOI assigned to each PDB entry, structures deposited in the PDB can be referenced and accessed in the same manner as other electronic publications.