A registry service to create links from specific articles, journals, or biological data in Entrez to resources on external web sites. Third parties can provide a URL, resource name, brief description of their web sites, and specification of the NCBI data from which they would like to establish links. The specification can be written as a valid Boolean query to Entrez or as a list of identifiers for specific articles or sequences. Entrez PubMed users can then select which external links are visible in their searches through the NCBI Cubby service (see above). (See the LinkOut chapter or web page.)