Where necessary, additional AC lines are used. Accession numbers are the primary means of identifying sequences provide a stable way of identifying entries from release to release. An accession number, however, always remains in the accession number list of the latest version of the entry in which it first appeared.
Accession numbers allow unambiguous citation of database entries.
One reason for allowing the existence of several accession numbers is to allow tracking of data when entries are merged or split. An accession number is dropped from the database only when the data to which it was assigned have been completely removed from the database.