PubMed Central

PubMed Central Help
Updated:  June 22, 2004

• PubMed Central Searching
•     Author
•     Journal
•     Automatic Term Mapping
•     Truncation
•     Phrase Searching
•     Additional Search Rules
•         Boolean Commands & Search Field Descriptions and Tags
•     Cookies
• Features Bar
•     Limits
•     Preview/Index
•     History
•     Clipboard
•     Details
• Documents
•     Display
•     Sort
•     Save
•     Print
• Links
• Other Services
•     E-Utilities
•     Journals Database
• Search Field Descriptions and Tags

Databases on the black menu bar are links to the other NCBI resources.

To search PubMed Central enter search terms in the query box, and press the Enter key or click Go.  The Features bar directly beneath the query box provides access to additional search options. The PubMed Central query box and Features bar are available from every screen, so you don't need to return to the homepage to enter a new search.

You may enter one or more terms (e.g., scientific collaboration trends) in the query box; PubMed Central automatically combines (ANDs) significant terms together using automatic term mapping. The terms are searched in various fields of the record. If your search includes Boolean operators (AND, OR, NOT), they must be in upper case, e.g., "acupuncture OR massage".

Once you click Go, PubMed Central will display your search results.  The query box displays your search terms as you entered them.

You can modify your current search by adding or eliminating terms in the query box or in Details.  If you applied Limits, the check box next to Limits will be marked, and a listing of your limit selections will be displayed. To turn off the existing limits, uncheck the Limits checkbox before running your next search.

Author
To search by an author's name, enter the name in the format of last name plus initials (no punctuation), e.g., smith a black dh. PubMed Central automatically truncates the author's name to account for varying initials and designations such as Jr. or 2nd. A name entered using this format will search in the author field. If only the author's last name is entered, PubMed Central searches the name in All Fields, except when the author name is found in the MeSH (National Library of Medicine's Medical Subject Headings) Translation table (e.g., Yang will search as Yin-Yang [MeSH] or Yang [Text Word].) To search for an author in the author field when only the last name is available qualify the author with the author search field tag [au], e.g., "yang [au]".

Note:

1. Use double quotes around the author's name with the author search field tag [au] to turn off the automatic truncation, e.g., "smith k" [au].
2. Use the [au] search tag if the author name is also a subject term, e.g., moran a [au]. The unqualified phrase moran a will search as "moran A"[Substance Name] OR moran a[Text Word].

Note:

1. If a journal is also a MeSH term (e.g., Critical Care), PubMed Central searches the unqualified term as MeSH. Qualify the journal title with the Journal Title search field tag, e.g., critical care [ta], to search for the journal.
2. Qualify single word journal titles using the Journal Title search field tag, e.g., scanning [ta], as PubMed Central will search unqualified single journal titles as MeSH terms (if applicable) or in All Fields.
3. If a journal includes parentheses or brackets, enter the name without the parentheses or brackets, e.g., search J Hand Surg [Am] as j hand surg am.

1. MeSH and Organism Translation Tables
The MeSH translation table contains MeSH Terms, the See-Reference mappings (also known as entry terms) for MeSH Heading terms, MeSH Subheadings, terms derived from the Unified Medical Language System (UMLS) that have equivalent synonyms or lexical variants in English, and the Supplementary Concept (Substance) Names and their synonyms. If a match is found in this translation table, the term will be searched as MeSH and as a Text Word. For example, if you enter "vitamin h" in the query box, PubMed Central will translate this search to: ("Biotin"[MeSH Terms] OR vitamin h[Text Word]) - vitamin h is an entry term for the MeSH term Biotin.

The Organism translation table contains the scientific and common names for the organisms associated with protein and nucleotide sequences.

Note:

Click on Details to verify how your terms are translated.

3. Phrase List
If no match is found, PubMed Central consults a Phrase List. Phrases on this list are generated from MeSH, the UMLS, and Supplementary Concept Substance Names, e.g., "multidrug transporter".

4. Author Index
If the phrase is not found in the above tables or list and is a word followed by one or two letters, then PubMed Central checks the Author index.

If no match is found?
PubMed Central breaks apart the phrase and repeats the above automatic term mapping process until a match is found. If there is no match, the individual terms will be combined (ANDed) and searched in All Fields.

See Also:

• Truncation
• Phrase Searching (forcing PubMed Central to search for a phrase)

Wildcard search for 'term*' used only the first 150 variations. Lengthen the root word to search for all endings.

Note:

Truncation turns off automatic term mapping and the automatic explosion of a MeSH term. For example, heart attack* will not map to the MeSH term, Myocardial Infarction or include any of the more specific terms, e.g., Myocardial Stunning; Shock, Cardiogenic.

Note:

1. If your search phrase is not on the list of recognized phrases, then the double quotes are ignored, and the phrase is processed using automatic term mapping. Your phrase may actually appear in the record data yet not be on the phrase list.
2. When you enclose a phrase in double quotes, PubMed Central will not perform automatic term mapping. For example, "health planning" will include citations that are indexed to the MeSH term, Health Planning, but will not include the more specific terms, e.g., Health Care Rationing, Health Care Reform, Health Plan Implementation and so on, that are typically included with the automatic MeSH mapping.

• Limits
• Additional Search Commands

• Boolean and Syntax
• Possessive Names
• Search Field Qualification
• Dates & Date Ranging
• Subsets

1. Boolean operators, AND, OR, NOT must be entered in upper case, e.g., vitamin c OR zinc.
2. PubMed Central processes all Boolean connectors in a left-to-right sequence. You can change the order by enclosing individual concepts in parentheses. The terms inside the set of parentheses will be processed as a unit and then incorporated into the overall strategy, e.g., common cold AND (vitamin c OR zinc).
3. If PubMed Central finds a phrase within a search strategy string that uses unqualified terms, it will automatically search the terms as a phrase rather than combining the individual words. For example, if you enter multidrug transporter in the query box, PubMed Central will search "multidrug transporter" as a phrase. If you do not want this automatic phrase parsing, enter each term separated by the Boolean operator AND, e.g., multidrug AND transporter.
4. Follow the search and syntax rules to enter search terms, specify the search field to be searched and the Boolean operators in your search statement:

Examples of Boolean Search Statements:

Find citations on DNA that were authored by C Campbell:

dna AND campbell c

Find articles that deal with the effects of heat or humidity on multiple sclerosis, where
these words appear in all fields in the citation:

(heat OR humidity) AND multiple sclerosis

Possessive Names as Part of Terms
If you are searching for a term named for an individual, be sure to include the full name of the term, e.g., "crohn's disease" or "coon's technique". The possessive form is not required, i.e., "crohn disease", can be used. Alternatively, use the non-possessive form of the term with the [tw] search tag, e.g., "hodgkin [tw]".

Search Field Qualification
Terms can be qualified using Search Field tags.

1. Add the search tag after the term, e.g., "neoplasms [mh]"
2. Search field tags must be enclosed in square brackets, "[mh]"

To enter a date range, insert a colon (:) between each date, e.g., "1993:1995 [dp]" or
"1997/01:1997/06 [pmcdat]".

Date Fields:

Date of Publication [pdat]
PMC Live Date [pmcdat] The date the citation first entered PubMed Central.
Electronic Publication Date [epubdat] The date of electronic publication.

Date ranging is also available from the fill-in-the-blank selection on the Limits screen.

Cookies
What is a "cookie"?
A "cookie" is information sent by a web site server (e.g., PubMed Central) to your computer. In the case of PubMed Central, it is information about your interactions that may be needed to perform a later function. Cookies allow PubMed Central to provide more interactive features such as Preview/Index, Clipboard, History, and the Cubby. Cookies placed by PubMed Central are removed from your computer after a set time period. There are many web sites that provide information about cookies. Netscape provides technical information.

How can I tell if my system accepts cookies?
Cookie-dependent features of PubMed Central include Preview/Index, History, Clipboard, and the Cubby. To use these features, your system must be able to accept cookies. If you still have problems using cookie-dependent features of PubMed Central even after enabling cookies possible reasons may include:

1. Cookies are blocked by your provider or institution: Check with your Internet provider and/or the system administrator at your institution to see if cookies can be accepted. Even if you have them enabled in your web browser, if they are blocked by your provider or institution (e.g., by a firewall, proxy server, etc.), cookie-dependent features of PubMed Central won't work.
2. Your computer's date & time settings are incorrect: Check your computer's time settings to ensure that they are correct.

Note:

If you select a limit and either run a search or move to another screen, a check will appear in a box next to Limits on the Features bar to indicate that limits have been selected. If you then run a search, the limits in effect will appear in the yellow bar above the Display button. To turn off the limits before you run your next search, uncheck the box.

See also:

• Search Field Qualification

• Preview the number of search results before displaying the citations.
• Refine search strategies by adding one or more terms one at a time.
• Add terms to a strategy from specific search fields.
• View and select terms from the Index to develop search strategies.

To search for terms from specific search fields use the Add Term(s) to Query text box. Select a search field from the All Fields pull-down menu and enter a term in the text box. Click AND, OR, or NOT to add the term to the query box with the appropriate search field tag, or click Preview to see the number of results.

The available Boolean operators are:

Intersection (AND) - only those citations that contain selected terms.
Union (OR) - citations that contain at least one of the selected terms.
Difference (NOT) - exclude citations with the selected term.

You can also use Index to select from a list of terms within a search field. Select a search field from the All Fields pull-down menu, enter a term in the box, and click Index. PubMed Central displays an alphabetic list of terms in the Index for the selected search field. The number of citations in PubMed Central that contain the term appears in parentheses to the right of the term. You can scroll up or down the list, or click Up or Down to move along in the Index.

Highlight a term by clicking on it. Then click the appropriate operator, AND, OR, NOT. The selected term will be added to the query box. To OR together multiple terms from an Index display and then add (or AND) them to your search, click on each term while holding down the Ctrl-key (PC) or the Command-key (Mac). When all the terms you want are highlighted, click the connector AND to add the terms (ORed together) to the query.

You may then continue to add additional terms from other search fields. Once your strategy is complete, click Preview to see the number of results, or click Go to display the citations.

Note:

1. Preview/Index displays the last three queries from History. Use History to review up to the last 100 queries.
2. PubMed Central processes all Boolean operators left to right. To change this order enclose search terms to be processed first in parentheses, e.g., common cold AND (vitamin c OR zinc). You may edit (e.g., add parentheses) in any query box. PubMed Central will automatically OR (and add parentheses) for multiple terms selected from the Index.
3. Preview uses cookies to keep a history of your searches. In order for you to use this feature your web browser must be set to accept cookies.

You can combine searches or add additional terms to an existing search by using the pound sign (#) before the search number, e.g., "#2 AND #6", or "#3 AND (drug therapy OR diet therapy)". Once you have entered a revised search strategy in the query box, click Go to view the search results. Click Clear History to remove all searches from the History screen and the Preview/Index screen.

Note:

1. The maximum number of queries held in History is 100. Once the maximum number is reached, PubMed Central will remove the oldest search from the History to add the most current search.
2. The Search History will be lost after one hour of inactivity on PubMed Central or any of the other Entrez databases. The Clear History button in History will also clear the Preview/Index history information.
3. PubMed Central will move a search statement number to the top of the History if the new search is the same as a previous search.
4. A separate Search History will be kept for each of the Entrez databases although the search statement numbers will be assigned sequentially for all databases.
5. PubMed Central uses cookies to keep a history of your searches. To use this feature, your web browser must be set to accept cookies.
6. History search numbers may not be continuous because some numbers are assigned to intermediate processes such as displaying a citation in another format.

Clipboard
The Clipboard gives you a place to collect selected citations from one search or several searches. After you add citations to the Clipboard, you may then want to use the print, save, or order buttons. The maximum number of items that can be placed in the Clipboard is 500. Once you have added items to the Clipboard, you can click on Clipboard from the Features bar to view your selections.

Add to Clipboard
To place an item in the Clipboard, click on the check box to the left of the citation, select Clipboard from the Send to pull-down menu, and then click the Send to button. Once you have added a citation to the Clipboard, the record number color will change to green.

Note:

1. If you select Clipboard from the Send to menu without selecting citations using the check box, PubMed Central will add all (up to 500 citations) of your search results to the Clipboard.
2. The Clipboard will be lost after 1 hour of inactivity on PubMed Central or any of the other Entrez databases.
3. PubMed Central uses cookies to add your selections to the Clipboard. To use this feature, your web browser must be set to accept cookies.
4. Citations in the Clipboard are represented by the search number #0 which may be used in Boolean search statements.

Note:

Your results may be on more than one page. PubMed Central will retain your selections from all pages of your results. To mark specific citations to save, click on the check box to the left of each citation and continue to page through your results. Use Save to save all the selected citations to a file.

• Printing

Details
Details lets you view your search strategy as it was translated using automatic term mapping and search rules and syntax. Also, from Details, you can save a search query or edit the search query and resubmit it. Details also includes error messages and notes.

Editing Your Search in Details
The Query box shows the actual search strategy and syntax used to run the search. Beneath this box, the Result number hyperlink displays the total number of matches for the current search. To return to the current search results screen, click this link or use the "Back" function of your Web browser. The Translations area details how each term was translated, and the User Query area shows the search terms as you entered them in the query box.

To edit the search strategy in the Query box, click in the box to add or delete terms and then click Search.

Saving a Search Strategy from Details
From the Details window, use the URL button to display the current search strategy as a URL and then bookmark the URL for future use.

To save a search strategy:

1. From the Details window, click on URL. You will return to the search results screen. The translated search strategy will be displayed in the query box, and this search strategy will also be embedded as part of the URL.
2. Next, use your Web browser's bookmark function to save the URL as a bookmark. After saving the bookmark, you may want to use your Web browser's edit functions to rename the bookmark.

Searches that were created using a search statement number in History (e.g., "#1 or #2 AND human[mh]") should not be saved using the URL feature, as the search statements represented by these numbers are lost when History is deleted.

The Show pull-down menu allows you to change the number of citations displayed on a single page up to a maximum of 500 items. To do this:

• From the Summary Page, click on the Show pull-down menu and select a number. To have all of the citations displayed on a single page, select a number higher than the total number of your search results.
• Click the Display button to redisplay your citations according to your selection.

PubMed Central citations are initially displayed in a summary format. You can choose to display other formats:

• Click on the Abstract, Full Text, PDF or PubLink hyperlink for a specific citation.
• All Citations -Select a display format from the Display pull-down menu and then click Display to view a different display or Links for all citations on the page.
• Selected Citations - Click on the boxes to the left of each author to select specific citations and then select a format or Links from the Display pull-down menu and click Display.

Display Order
Citations in PubMed Central are displayed in relevancy order.

Sort
To change the citation order display from relevancy select a sort field from the Sort pull-down menu and click Display. Publication Date sorts the most recent citations first, the secondary sort is journal. Electronic Publication Date sorts by most recent online publication date. Journal sorts A to Z, the secondary sort is publication date. PMC Live Date sorts by date available online.

Save
The Save function creates a text file of citations on your computer. The Send to File selection is available on screens with search results. You can save entire search results or selected citations. If you don't select specific citations before selecting File and clicking on the Send to button, you will create a file on your computer that will contain the entire search results up to a maximum of 10,000 items.

You can also save citations collected from multiple searches by using the Send to Clipboard feature. The Clipboard holds a maximum of 500 items. When you save a file, your browser will prompt you where this file should be placed on your computer, and also give you the opportunity to rename the file.

Saving Entire Search Results:

1. The default for the Save feature is to save the entire retrieval, up to 10,000 items, unless you specifically select citations. For example, if you use the Send to File on a screen displaying 1-20 items of 30, your saved file will contain all 30 citations.
2. Use the Display pull-down menu to select a format.
3. Click Send to File and your items will be saved in this format as plain text.

1. Use the Display pull-down menu to select a format.
2. Click in the check boxes next to each citation you want to save.
3. You can move to other pages within the search results to make more selections.
4. Click Send to File and your selected items will be saved in the Display format as plain text.

1. Use the Send to Clipboard selection to add either entire search results or selected items to the Clipboard.
2. The Clipboard can contain up to 500 items.
3. After adding citations to the Clipboard, click on the Clipboard link to go to that screen.
4. Use the Display pull-down menu to select a format.
5. Choose File and click Send to and your items will be saved in this format as plain text.

1. Saving a large retrieval may take several minutes.
2. To save citations in HTML format, use the Save as... function of your browser. Change the file extension to html. You will only save the citations displayed on the screen so you may wish to use the Show function and Text button to adjust your display as needed.

Printing Selected Citations:

You may want to print selected items from one or more searches on a single page. You can do this by collecting them in the Clipboard:

1. Click in the box next to each citation you want and then Send to Clipboard. The Clipboard can hold up to 500 items.
2. When you are ready to print, click on the Clipboard link to go there.
3. Within the Clipboard, display the citations in the format and sort order you want to print.
4. Use your browser's print function to print the page.

You may also wish to display your citations as Text to strip the sidebar menu and toolbars prior to printing your results.

• Domain - The Conserved Domain Database is used to identify the conserved domains present in a protein sequence.
• Genome - Provides access to records and graphic displays of entire genomes and chromosomes for megabase sequences obtained from large-scale sequencing of genomes and chromosomes.
• ProbeSet - Gene expression data repository and online resource for the retrieval of gene expression data from any organism or artificial source.
• Nucleotide - DNA sequences from GenBank, EMBL, and DDBJ.
• OMIM - Catalog of human genes and genetic disorders.
• PopSet - The PopSet database contains aligned sequences submitted as a set from a population, phylogenetic or mutation study describing such events as evolution and population variation.
• Protein - Amino acid (protein) sequences from Swiss-Prot, PIR, PRF, PDB, and translated protein sequences from the DNA sequences databases.
• PubMed - PubMed provides access to bibliographic information which includes MEDLINE.
• Referenced Articles - The record is cited by a full-text article in PubMed Central.
• SNP - dbSNP is a database of Single Nucleotide Polymorphisms.
• Structure - The Molecular Modeling Database (MMDB) contains 3-dimensional structures determined by X-ray crystallography and NMR spectroscopy.
• Taxonomy - Contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence.

The Journals database can be searched using the journal title, the MEDLINE abbreviation, the NLM ID, the ISO (International Organization for Standardization) abbreviation, and the print or electronic ISSNs (International Standard Serial Numbers). The database includes the journals in all Entrez databases.

Note:

1. Click the journal title from the Summary display or choose Full from the Display pull-down menu to view additional information.
2. To limit journal searches to PubMed journals, use the "Only PubMed journals" selection in Limits or include the PubMed journal subset (i.e., "journals pubmed [sb]") in your search strategy.
3. Unqualified journal terms are searched in all journal fields. The following qualifiers are available to limit your search to a specific field: [ISO Abbr] [ISSN] [MEDLINE Abbr] [NLM ID], and [Title].
4. The Links menu includes links to the Entrez databases in which citations to the journal are found. Select a database under the Links menu to retrieve records for that journal.
5. Click the NLM ID to display journal information in NLM's online catalog Locatorplus.
6. If a journal includes parentheses or brackets, e.g., "J Hand Surg [Am]", enter the abbreviation or title without the special characters, j hand surg am.

• Affiliation [ad] - Includes the institutional affiliation and address (including email address) of the authors of the article as it appears in the journal. This field can be used to search for work done at specific institutions (e.g., "cleveland [ad] AND clinic [ad]").
• All Fields [all] - Includes all searchable PubMed Central fields. However, only terms where there is no match found in one of the Translation tables or Indexes via the Automatic Term Mapping process will be searched in All Fields. PubMed Central ignores stopwords from search queries.
• Abstract [ab] - Includes all words and numbers in the abstract of an article.
• Accession [accn] - Accession numbers from the GenBank sequence database that are cited in PMC articles.
• Author [au] - PubMed Central citations do not list the full author name. The format to search for an author is: last name followed by a space and up to the first two initials followed by a space and a suffix abbreviation, if applicable, all without periods or a comma after the last name (e.g., "fauci as or o'brien jc jr"). Initials and suffixes may be omitted when searching. PubMed Central automatically truncates on an author's name to account for varying initials, e.g., o'brien j [au] will retrieve o'brien ja, o'brien jb, o'brien jc jr, as well as o'brien j. To turn off this automatic truncation, enclose the author's name in double quotes and qualify with [au] in brackets, e.g., "o'brien j" [au] to retrieve just o'brien j.
• Body - All Words [articlebody] - Includes all words and numbers in the body of a article except for the Abstract and References.
• Body - Key Terms [body - key terms] - Key terms from the article body.
• EC/RN Number [rn] - Number assigned by the Enzyme Commission to designate a particular enzyme or by the Chemical Abstracts Service (CAS) for Registry Numbers.
• Electronic Publication Date [epubdat] - The date of the electronic publication. Dates or date ranges must be entered using the format YYYY/MM/DD [edat], e.g. 1998/04/06 [edat] . The month and day are optional (e.g., 1998 [edat] or 1998/03 [epubdat]). To enter a date range, insert a colon (:) between each date (e.g., 1996:1997 [epubdat] or 1998/01:1998/04 [epubdat])
• Figure/Table Caption [figure/table caption] - Includes all words and numbers in the figure and table captions of an article.
• Filter [filter] - Technical tags used by PubMed Central to qualify citations.
• Issue [ip] - The number of the journal issue in which the article is published.
• Journal [ta] - The journal title abbreviation, full journal title, or ISSN number (e.g., J Biol Chem, Journal of Biological Chemistry, 0021-9258). The Journal Database is also available to look up the full name, abbreviation, and ISSN number of a journal. If a journal title contains parentheses or brackets, enter the name without the parentheses or brackets, e.g., enter J Hand Surg [Am] as j hand surg am.
• MeSH Major Topic [majr] - A MeSH term that is one of the main topics discussed in the article. See MeSH Terms below.
• MeSH Subheadings [sh] - MeSH Subheadings are used with MeSH terms to help describe more completely a particular aspect of a subject. For example, the drug therapy of asthma is displayed as asthma/drug therapy, see MeSH/Subheading Combinations. The MeSH Subheading field allows users to "free float" Subheadings, e.g., "hypertension [mh] AND toxicity [sh]". MeSH Subheadings automatically include the more specific Subheading terms under the term in a search. To turn off this automatic feature, use the search syntax [sh:noexp], e.g., "therapy [sh:noexp]". In addition, you can enter the MEDLINE two-letter MeSH Subheading abbreviations rather than spelling out the Subheading, e.g., "dh [sh] = diet therapy [sh]".
• MeSH Terms [mh] - NLM's Medical Subject Headings controlled vocabulary of biomedical terms is used to describe the subject of each journal article in MEDLINE. MeSH contains more than 19,000 terms and is updated annually to reflect changes in medicine and medical terminology. MeSH terms are arranged hierarchically by subject categories with more specific terms arranged beneath broader terms.

  Skilled subject analysts examine journal articles and assign to each the most specific MeSH terms applicable -- typically ten to twelve. Applying the MeSH vocabulary ensures that articles are uniformly indexed by subject, whatever the author's words.

  Notes on MeSH Terms and Major MeSH Topic search fields:

  1. To search the term as a MeSH term, it must be qualified using the search field tags, e.g., [mh] for MeSH Terms or [majr] for MeSH Major Topic. Any term so qualified is checked against the MeSH Translation table. The MeSH translation table includes concepts that may map to two or more MeSH terms. To turn off this mapping, enclose the MeSH term in double quotes and qualify with [mh], e.g., "cold" [mh].
  2. MeSH terms are arranged hierarchically by subject categories with more specific terms arranged beneath broader terms. MeSH terms in PubMed Central automatically include the more specific MeSH terms in a search.
  3. MeSH/Subheading Combinations: To directly attach MeSH Subheadings, use the format MeSH Term/Subheading, e.g., "neoplasms/diet therapy [majr]". You may also use the MEDLINE two-letter MeSH Subheading abbreviations, e.g., "neoplasms/dt [mh]", etc. Only one Subheading may be attached to a MeSH term directly. For a MeSH/Subheading combination, PubMed Central always includes the more specific terms arranged beneath broader terms for the MeSH term and also includes the more specific terms arranged beneath broader Subheadings. The broader Subheading or one of its indention's will be attached directly to the MeSH term or one of its indention's. For example, hypertension/therapy also retrieves hypertension/diet therapy; hypertension/drug therapy; hypertension, malignant/therapy; hypertension, malignant/drug therapy and so on as well as hypertension/therapy.
  4. To turn off the automatic inclusion of the more specific terms, use the syntax [field:noexp], e.g., "hypertension [mh:noexp]", or "hypertension [majr:noexp]", or "hypertension/therapy [mh:noexp]". The latter example turns off the more specific terms in both parts, searching for only the one Subheading therapy attached directly to only the one MeSH term hypertension.
  
  If a MeSH term contains parentheses, enter the name without the parentheses, e.g., enter the MeSH term Earth (Planet) as earth planet [mh].

• Methods - Key Terms [methods - key terms] - Key terms from the "methods" section of the article abstract.
• Organism [orgn] - Contains the scientific and common names for the organisms in the NCBI taxonomy database that are found in PMC articles. The NCBI taxonomy database only includes species that are represented in the public sequence databases, so many of the organisms mentioned in PMC articles will not be found in this index. To cut down on false hits, matches to scientific names are required to add PMC articles to this index - common names in this field will only retrieve articles that mention the corresponding scientific name. By default the PMC organism index is 'unexploded' - the query "Mammalia[orgn]" will only retrieve articles that explicitly mention the Mammalia. The query "Mammalia[orgn:exp]" will retrieve articles that mention any of the species of mammals.
• Organism Unsynonymized [orgn_nosyn] - Organism names that are identified as Synonyms in the NCBI taxonomy database will retrieve the same set of articles in the PMC Organism index (e.g "Danio rerio[orgn]" = "Brachydanio rerio[orgn]"). The Organism unsynonymized index will only retrieve articles that use the name in question.
• Pagination [pg] - Enter only the first page number that the article appears on. The citation will display the full pagination of the article, but this field is searchable using only the first page number.
• PMC Live Date [pmcdat] - Date the citation was first available in PubMed Central. Dates or date ranges must be entered using the format YYYY/MM/DD, e.g. "1998/04/06 [pmcdat]". The month and day are optional (e.g., "1998 [pmcdat]" or "1998/03 [pmcdat]"). To enter a date range, insert a colon (:) between each date (e.g., "1996:1997 [pmcdat]" or "1998/01:1998/04 [pmcdat]").
• Publication Date [dp] - The date that the article was published. Dates or date ranges must be searched using the format YYYY/MM/DD, e.g. "1998/03/06 [dp]". The month and day are optional (e.g., "1998 [dp]" or "1998/03 [dp]"). To enter a date range, insert a colon (:) between each date (e.g., "1996:1998 [dp]" or "1998/01:1998/04 [dp]").

  Note:

  Journals vary in the way the publication date appears on an issue. Some journals include just the year, whereas others include the year plus month or year plus month plus day. And, some journals use the year and season (e.g., Winter 1997). The publication date in the citation is recorded as it appears in the journal. It is recommended that you search only by year (e.g., 1996 [dp] or 1995:1997 [dp]).

• Reference [reference] - Includes words and numbers in the titles of the journal references.
• Reference Author [reference author] - Includes reference author names. PubMed Central references do not list the full author name. The format to search for an author is: last name followed by a space and up to the first two initials followed by a space and a suffix abbreviation, if applicable, all without periods or a comma after the last name (e.g., "fauci as" or "o'brien jc jr"). Initials and suffixes may be omitted when searching. PubMed Central automatically truncates on an author's name to account for varying initials, e.g., "o'brien j [au]" will retrieve o'brien ja, o'brien jb, o'brien jc jr, as well as o'brien j. To turn off this automatic truncation, enclose the author's name in double quotes and qualify with [au] in brackets, e.g., "o'brien j" [au] to retrieve just o'brien j.
• Section title [section title] - Includes words and numbers in the journal article section title.
• Substance Name [nm] - The name of a chemical discussed in the article. Synonyms to the Supplementary Concept Substance Name will automatically map when qualified with [nm]. This field was implemented in mid-1980. Many chemical names are searchable as MeSH terms before that date.
Text Words [tw] - Includes all words and numbers in the title and abstract as well as MeSH terms, MeSH Subheadings, and chemical substance names.
• Title [ti] Words and numbers included in the title of a citation.
• Unique Identifiers [uid] - PubMed Central Unique Identifier. To search for the UID type in the number with the search field tag [uid].
• Volume [vi] - The number of the journal volume in which an article is published.
  
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