Entrez is the search interface available at NCBI to find entries in their databases using text queries. The main Entrez search page is a good place to start if you're not sure which database contains the information that you're looking for. 

Search Entrez Global Query. 

For example, searching for cancer will pull up all records in each database that have the text "cancer" anywhere in the record. You can also retrieve information about genes by searching for the gene symbol. 

Combine search terms using Boolean operators.

Boolean operators allow you to combine search terms: 

• AND: Finds documents that contain both terms.
• OR: Finds documents that contain either term. 
• NOT: Finds documents that contain the term on the left but not the term on the right.

Make sure the operators are upper case! Some databases won't recognize them as Boolean operators if they are lower case. You can also use parentheses to group terms. Remember: AND is always evaluated first!

For example, suppose you're looking for information about the brca1 gene's role in cancer:

brca1 AND cancer

Or maybe you want to know about cancers that involve brca1or P53:

(brca1 OR P53) AND cancer

Or maybe you want to find out information about P53 that isn't related to cancer:

P53 NOT cancer

Field Tags

Individual databases have field tags, which allow you to specify which allow you to search for your query only in specific fields. For example, searching PubMed for black[au] would search for "black" only in the author field, and not in any other field. Compare those results to searching PubMed for the word black.You can use these field tags in the global Entrez search box, but you won't get results from databases that don't use these field tags. 

See pages for individual databases to see useful field tags for each database.

Searching Entrez Global Query returns a page with a list of each database, and the number of results from each database that match your query. To see the results in each database, click on the name of the database. See the pages for individual databases for more information about interpreting the results.