Accession numbers are identifiers for a sequence, for example P123456. They can have version numbers if suffixed with a "." and a number, for example P123456.2. This aids distinguishing between older and newer versions of a sequence, and t
Accession numbers are identifiers for a sequence, for example P123456. They can have version numbers if suffixed with a "." and a number, for example P123456.2. This aids distinguishing between older and newer versions of a sequence, and to track which actual sequence was used in an analysis.
NCBI Reference sequences have their own syntax.
Accessions are allocated in batches to the different sequence repositories DDBJ, EMBL Database, and NCBI. Table 1 shows the format of some unversioned accession numbers.
Table 1: Some Accession Number Formats
| Database | Regular Expression | Perl Regular Expression |
| RefSeq | [:alpha]{2}_[:digit]{6,9} or NZ_[:alpha]{4} [:digit]{6,9} | [A-Z]{2}_\d{6,9} or NZ_[A-Z]{4}\d{6,9} |
| Swissprot | [OPQ][:digit][:alnum]{3}[:digit] | |
| GenBank/EMBL/DDBJ | [:alpha][:digit]{5} or [:alpha]{2}[:digit]{6} | [A-Z]\d{5} or [A-Z]{2}\d{6} |
| PRF | [:digit]{6,7} [:alpha] | \d{6,7}[A-Z] |
| PDB | [:digit][:alpha]{3} | \d[A-Z]{3} |
| MMDB | [:digit]{4} | \d{4} |
| GenBank GI | [:digit]{5,} | \d{5,} |
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