Qualifier
db_xref
Definition database cross-reference: pointer to related information in another database.
Value Format
"<database:identifier>" where database is
the name of the database containing related information, and 
identifier is the internal identifier of the related information
according to the naming conventions of the cross-referenced 
database.
Example /db_xref="UniProtKB/Swiss-Prot:P28763"
Comment the complete list of allowed database types is kept at http://www.insdc.org/db_xref.html