Sequence Revision History
GenBank can show the revision history of a sequence. The revision history shows the various GI numbers, version numbers, and update dates for sequences that appeared in a specific GenBank record.
To see the revision history of a sequence, append report=girevhist
to the record's URL. For example, accession U46667's revision
history's URL is http://www.ncbi.nlm.nih.gov/nuccore/U46667 ?report=girevhist.
Looking at that record, note that the original gi number for the nucleotide sequence, 2734632, does not have a corresponding version number. The version number is missing because it was removed from the database (and replaced by 3172140) before the new accession.version system was implemented in Feb. 1999. At that time, each sequence in the GenBank/EMBL/DDBJ database received a version number of 1, even if they had been updated in the past.
In addition, when a GenBank record contains an updated sequence,
the COMMENT field will contain a cross-reference to the gi number of the
earlier sequence; e.g., see this excerpt from
U46667:
COMMENT [WARNING] On Jun 2, 1998 this sequence was replaced by gi:3172140.
If you follow the link for that earlier gi number, Entrez will display
that version of the GenBank record. Similarly, the COMMENT field of
the older version will have a warning that the sequence has been
updated, and will contain a cross-reference to the newer version.
Read more about Sequence and Version Numbers.