Enumerations for _Related_entries.Database_name (A closed enumeration) 
BMCD 'Biological Macromolecule Crystallization Database' 
BMRB 'BioMagResBank' 
EMDB 'Electron Microscopy Volume Data Bank' 
NDB 'Nucleic Acid Database' 
PDB 'Protein Databank' 
TargetDB 'Target registration database'