The accession number is a unique identifier assigned to a record in sequence databases such as GenBank. Several NCBI databases use the format [alphabetical prefix][series of digits]. A change in the record in some databases (e.g. GenBank) is tracked by an integer extension of the accession number, an Accession.version identifier. The initial version of a sequence has the extension “.1”. When a change is made to a sequence in a GenBank record, the version extension of the Accession.version identifier is incremented. For the sequence NM_000245.3, “.3” indicates that the record has been updated twice. The accession number for the record as a whole remains unchanged, and will always retrieve the most recent version of the record; the older versions remain available under the original Accession.version identifiers.