Each record in an NCBI database carries a unique record identifier (UID). The record’s UID unambiguously differentiates the record from all other records in the database. UIDs have names that indicate the associated database. For example, UIDs in the PubMed database are called PMIDs, and those for PubChem Compound records are called Compound CIDs.
UIDs most often consist only of a series of digits. Within a record, you will see the UID preceded by its name. Generically, it will look like this:

[UID name] [:] [series of digits]

For example, in PubMed you can see PMID: 35771962 and in PubChem you can see Compound CID: 2244.
 

What can you do with UIDs?

You can use UIDs to:

• Search for the exact records in a database on the web.
• Generate lists of interesting records from your text-search results.
• Access database records through various programming applications, such as E-utilities (the Entrez API).
• Establish links (URLs) to the records.
• Refer to the records in your communications/publications (for example, referring to Gene IDs is more accurate and precise than using gene symbols).

How can you search NCBI web with UIDs?

For targeted results:

• Start on the NCBI home page and select the database that you want to search from the pull-down menu.
• Enter one or more UIDs (the digits only) in the search box. Separate multiple UIDs with spaces or commas.
• Run the search.

If you search All Databases with a UID (example), you will likely obtain hits to several databases as UIDs are not unique between the databases. Subsequently, you will need to select the database of your interest.
 

Generally, NCBI does not support searching a database with non-native UIDs. There are a few exceptions, for example:

• You can use Taxonomy IDs to find Nucleotide and Protein sequences belonging to an organism. For example, the following search term retrieves sequences belonging to the American beaver with Taxonomy ID 51338:  txid51338[Organism]  
• You can also search PubMed with PubMed Central (PMC) UIDs. You need to keep the “PMC” prefix. For example, PMC140671 identifies a full-text article on a mammalian protein. By searching PubMed with PMC140671, you will retrieve the corresponding citation for the article, listed under PMID: 12482959.

For other possible exceptions, check the Advanced search in the database of interest to see what search fields/identifiers it supports.
 

Where can you learn more?

If you are working with NCBI sequence- and molecular data, you will encounter unique identifiers that are called accession numbers.  We describe these in several knowledge articles:

• What are accession numbers at NCBI?
• What are GenBank accession numbers and what information is embedded in their format?
• What are NCBI Reference Sequence (RefSeq) accession numbers and what information is embedded in their format?
• What are genome assembly accession numbers at NCBI?