The document outlines the usage of the NCBI E-utilities API for programmatically accessing data from the NCBI databases through web services. It describes the 8 E-utilities services including ESearch, ESummary, EFetch, ELink and provides examples of how to use them in analysis pipelines. For instance, it shows how to find related human genes to articles on osteosarcoma by searching PubMed, linking to genes, and summarizing the gene data. It also demonstrates integrating PubMed and GEO Dataset searches to find cancer copy number articles associated with specific microarray platforms.

1. NCBI API – Integration into
analysis code
QBRC Tech Talk
Jiwoong Kim

2. Outlines
• Introduction
• Usage Guidelines of the E-utilities
• Sample Applications of the E-utilities

3. NCBI & Entrez
• The National Center for
Biotechnology Information
advances science and health by
providing access to biomedical
and genomic information.
• Entrez is NCBI’s primary text
search and retrieval system
that integrates the PubMed
database of biomedical
literature with 39 other
literature and molecular
databases including DNA and
protein sequence, structure,
gene, genome, genetic
variation and gene expression.

4. E-utilities
• Entrez Programming Utilities
– The Entrez Programming Utilities (E-utilities) are a set of
eight server-side programs that provide a stable interface
into the Entrez query and database system at the NCBI.
– The E-utilities use a fixed URL syntax that translates a
standard set of input parameters into the values necessary
for various NCBI software components to search for and
retrieve the requested data.
E-utilitiesURL XML, FASTA, Text …
Input Output

5. Usage Guidelines and Requirements
• Use the E-utility URL
– baseURL: http://eutils.ncbi.nlm.nih.gov/entrez/eutils/ …
– Python urllib/urlopen, Perl LWP::Simple, Linux wget, …
• Frequency, Timing and Registration of E-utility URL Requests
– Make no more than 3 requests per second → sleep(0.5)
– Run large jobs on weekends or between 5 PM and 9 AM EST
– Include &tool and &email in all requests
• Minimizing the Number of Requests
– &retmax=500
• Handling Special Characters Within URLs
– Space → +, " → %22, # → %23

6. ESearch

7. ESearch (text searches)
• Responds to a text query with the list of matching UIDs in a
given database (for later use in ESummary, EFetch or ELink),
along with the term translations of the query.
• Syntax: esearch.fcgi?db=<database>&term=<query>
– Input: Entrez database (&db); Any Entrez text query (&term)
– Output: List of UIDs matching the Entrez query
• Example: Get the PubMed IDs (PMIDs) for articles about
osteosarcoma
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubmed&
term=%22osteosarcoma%22[majr:noexp]

8. ESummary
ESearch
UIDs
EFetch
UID

9. ESummary
(document summary downloads)
• Responds to a list of UIDs from a given database with the
corresponding document summaries.
• Syntax: esummary.fcgi?db=<database>&id=<uid_list>
– Input: List of UIDs (&id); Entrez database (&db)
– Output: XML DocSums
• Example: Download DocSums for these PubMed IDs:
24450072, 24333720, 24333432
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esummary.fcgi?db=pubme
d&id=24450072,24333720,24333432

10. EFetch
ELink

11. EFetch (data record downloads)
• Responds to a list of UIDs in a given database with the
corresponding data records in a specified format.
• Syntax:
efetch.fcgi?db=<database>&id=<uid_list>&rettype=<retrieval
_type>&retmode=<retrieval_mode>
– Input: List of UIDs (&id); Entrez database (&db); Retrieval type
(&rettype); Retrieval mode (&retmode)
– Output: Formatted data records as specified
• Example: Download the abstract of PubMed ID 24333432
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pubmed&i
d=24333432&rettype=abstract&retmode=text

12. ELink (Entrez links)
• Responds to a list of UIDs in a given database with either a list
of related UIDs (and relevancy scores) in the same database
or a list of linked UIDs in another Entrez database
• Checks for the existence of a specified link from a list of one
or more UIDs
• Creates a hyperlink to the primary LinkOut provider for a
specific UID and database, or lists LinkOut URLs and attributes
for multiple UIDs.

13. ELink (Entrez links)
• Syntax:
elink.fcgi?dbfrom=<source_db>&db=<destination_db>&id=<u
id_list>
– Input: List of UIDs (&id); Source Entrez database (&dbfrom);
Destination Entrez database (&db)
– Output: XML containing linked UIDs from source and destination
databases
• Example: Find one set/separate sets of Gene IDs linked to
PubMed IDs 24333432 and 24314238
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubme
d&db=gene&id=24333432,24314238
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubme
d&db=gene&id=24333432&id=24314238

14. EGQuery

15. EGQuery (global query)
• Responds to a text query with the number of records
matching the query in each Entrez database.
• Syntax: egquery.fcgi?term=<query>
– Input: Entrez text query (&term)
– Output: XML containing the number of hits in each database.
• Example: Determine the number of records for mouse in
Entrez.
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/egquery.fcgi?term=mouse[
orgn]&retmode=xml

16. ESpell

17. ESpell (spelling suggestions)
• Retrieves spelling suggestions for a text query in a given
database.
• Syntax: espell.fcgi?term=<query>&db=<database>
– Input: Entrez text query (&term); Entrez database (&db)
– Output: XML containing the original query and spelling suggestions.
• Example: Find spelling suggestions for the PubMed query
"osteosacoma".
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/espell.fcgi?term=osteosac
oma&db=pmc

18. EInfo (database statistics)
• Provides the number of records indexed in each field of a
given database, the date of the last update of the database,
and the available links from the database to other Entrez
databases.
• Syntax: einfo.fcgi?db=<database>
– Input: Entrez database (&db)
– Output: XML containing database statistics
• Example: Find database statistics for Entrez Protein.
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/einfo.fcgi?db=protein

19. EPost (UID uploads)
• Accepts a list of UIDs from a given database, stores the set on
the History Server, and responds with a query key and web
environment for the uploaded dataset.
• Syntax: epost.fcgi?db=<database>&id=<uid_list>
– Input: List of UIDs (&id); Entrez database (&db)
– Output: Web environment (&WebEnv) and query key (&query_key)
parameters specifying the location on the Entrez history server of the
list of uploaded UIDs
• Example: Upload five Gene IDs (7173, 22018, 54314, 403521,
525013) for later processing.
– http://eutils.ncbi.nlm.nih.gov/entrez/eutils/epost.fcgi?db=gene&id=71
73,22018,54314,403521,525013

20. Application 1
• Find related human genes to articles searched for non-
extended MeSH term "Osteosarcoma" (PubMed → Gene)
1. http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=pubme
d&term=%22osteosarcoma%22[majr:noexp]&usehistory=y
2. http://eutils.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubm
ed&db=gene&query_key=1&WebEnv=NCID_1_220057266_130.14.
18.34_9001_1396281951_1196950266&term=%22homo+sapiens%
22[organism]&cmd=neighbor_history
3. http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esummary.fcgi?db=gene
&query_key=3&WebEnv=NCID_1_220057266_130.14.18.34_9001_
1396281951_1196950266

21. Application 1
• Find related human genes to articles searched for non-
extended MeSH term "Osteosarcoma" (PubMed → Gene)
– ftp://ftp.ncbi.nlm.nih.gov/gene/DATA/gene2pubmed.gz
• It can be used instead of "ELink".
– ftp://ftp.ncbi.nlm.nih.gov/gene/DATA/gene_info.gz
• It can be used instead of "ESummary".

22. Application 2
• Find nucleotide sequences of "Burkholderia cepacia complex"
and download in GenBank format
1. http://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?db=nuccor
e&term=%22burkholderia+cepacia+complex%22[organism]&usehist
ory=y
2. http://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=nuccore
&query_key=1&WebEnv=NCID_1_264773253_130.14.22.215_9001
_1396244608_457974498&rettype=gb&retmode=text

23. Application 3
• Find "cancer copy number" articles with "Affymetrix Genome-Wide Human SNP Array"
platform GEO Datasets
cancer "copy number"
esearch.fcgi?db=pubmed
Affymetrix "Genome-Wide" Human "SNP Array" AND gpl[Filter]
esearch.fcgi?db=gds
esummary.fcgi?db=pubmed
WebEnv, query_key
esummary.fcgi?db=gds
WebEnv, query_key
GPL9704
GPL8226
GPL6804
GPL6801
elink.fcgi?dbfrom=pubmed&db=gds esearch.fcgi?db=gds
Parsing
Result table
Common
PubMed title

24. "cancer copy number" articles
"Affymetrix Genome-Wide Human SNP Array"
platform GEO Datasets

25. Application 3
• Find "cancer copy number" articles with "Affymetrix Genome-Wide Human SNP Array"
platform GEO Datasets
cancer "copy number"
esearch.fcgi?db=pubmed
Affymetrix "Genome-Wide" Human "SNP Array" AND gpl[Filter]
esearch.fcgi?db=gds
esummary.fcgi?db=pubmed
WebEnv, query_key
esummary.fcgi?db=gds
WebEnv, query_key
GPL9704
GPL8226
GPL6804
GPL6801
elink.fcgi?dbfrom=pubmed&db=gds esearch.fcgi?db=gds
Parsing
Result table
Common
PubMed title

26. Application 3
• Find "cancer copy number" articles with "Affymetrix Genome-Wide Human SNP Array"
platform GEO Datasets

27. Application 3
• Find "cancer copy number" articles with "Affymetrix Genome-Wide Human SNP Array"
platform GEO Datasets
cancer "copy number"
esearch.fcgi?db=pubmed
Affymetrix "Genome-Wide" Human "SNP Array" AND gpl[Filter]
esearch.fcgi?db=gds
esummary.fcgi?db=pubmed
WebEnv, query_key
esummary.fcgi?db=gds
WebEnv, query_key
GPL9704
GPL8226
GPL6804
GPL6801
elink.fcgi?dbfrom=pubmed&db=gds esearch.fcgi?db=gds
Parsing
Result table
Common
PubMed title

28. Application 3
• Find "cancer copy number" articles with "Affymetrix Genome-Wide Human SNP Array"
platform GEO Datasets

29. Make custom scripts with XML-parser

30. EBot
• EBot is an interactive web tool that first allows
users to construct an arbitrary E-utility
analysis pipeline and then generates a Perl
script to execute the pipeline. The Perl script
can be downloaded and executed on any
computer with a Perl installation. For more
details, see the EBot page linked above.
– http://www.ncbi.nlm.nih.gov/Class/PowerTools/e
utils/ebot/ebot.cgi

31. Entrez Direct
• E-utilities on the UNIX Command Line
• Download from ftp://ftp.ncbi.nih.gov/entrez/entrezdirect/
• Entrez Direct Functions
– esearch performs a new Entrez search using terms in indexed fields.
– elink looks up neighbors (within a database) or links (between databases).
– efilter filters or restricts the results of a previous query.
– efetch downloads records or reports in a designated format.
– xtract converts XML into a table of data values.
– einfo obtains information on indexed fields in an Entrez database.
– epost uploads unique identifiers (UIDs) or sequence accession numbers.
– nquire sends a URL request to a web page or CGI service.
• Entering Query Commands
– esearch -db pubmed -query "opsin gene conversion" | elink -related

32. Links
• References
– Entrez Programming Utilities Help
• http://www.ncbi.nlm.nih.gov/books/NBK25501/
– Entrez Help
• http://www.ncbi.nlm.nih.gov/books/NBK3836/
• Useful Links
– Entrez Unique Identifiers (UIDs) for selected databases
• http://www.ncbi.nlm.nih.gov/books/NBK25497/table/chapter2.chapter2_table1/?r
eport=objectonly
– Valid values of &retmode and &rettype for EFetch (null = empty string)
• http://www.ncbi.nlm.nih.gov/books/NBK25499/table/chapter4.chapter4_table1/?r
eport=objectonly
– The full list of Entrez links
• http://eutils.ncbi.nlm.nih.gov/entrez/query/static/entrezlinks.html

33. NCBI databases
• Literature: PubMed, PubMed Central, NLM Catalog, MeSH, Books, Site
Search
• Health: PubMed Health, MedGen, GTR, dbGaP, ClinVar, OMIM, OMIA
• Organisms: Taxonomy
• Nucleotide Sequences: Nucleotide, GSS, EST, SRA, PopSet, Probe
• Genomes: Genome, Assembly, Epigenomics, UniSTS, SNP, dbVar,
BioProject, BioSample, Clone
• Genes: Gene, HomoloGene, UniGene, GEO Profiles, GEO DataSets
• Proteins: Protein, Conserved Domains, Protein Clusters, Structure
• Chemicals: PubChem Compound, PubChem Substance, PubChem BioAssay
• Pathways: BioSystems

34. E-utilities
• Eight server-side programs
– ESearch : Searching a Database
– EPost : Uploading UIDs to Entrez
– ESummary : Downloading Document Summaries
– EFetch : Downloading Full Records
– ELink : Finding Related Data Through Entrez Links
– EInfo : Getting Database Statistics and Search Fields
– EGQuery : Performing a Global Entrez Search
– ESpell : Retrieving Spelling Suggestions

35. Sample Applications of the E-utilities
• Basic pipelines
– ESearch - ESummary/EFetch
– EPost - ESummary/EFetch
– ELink - ESummary/Efetch
– ESearch - ELink - ESummary/EFetch
– EPost - ELink - ESummary/EFetch
– EPost - ESearch
– ELink - ESearch

36. Application 3
• Find "cancer copy number" articles with "Affymetrix Genome-Wide Human SNP Array"
platform GEO Datasets
1. tr 'n' 't' < cancer_copy_number.pubmed_result.txt | sed 's/tt/n/g' | sed 's/^t[0-9]*: //' | sed 's/t/ /g' >
cancer_copy_number.pubmed_result.oneLine.txt
2. sed 's/^.* PubMed *PMID: *//' cancer_copy_number.pubmed_result.oneLine.txt | sed 's/; .*//' | sed 's/.$//' >
cancer_copy_number.pubmed_ids.txt
3. for id in $(cat cancer_copy_number.pubmed_ids.txt); do perl ~/scripts/elink.pl pubmed gds $id pubmed_gds | sed
"s/^/$idt/"; done > cancer_copy_number.pubmed_gds_ids.txt
4. awk -F't' '($1 == "Platform")' Affymetrix_Genome-Wide_Human_SNP_Array.gds_result.txt | cut -f2 | sed
's/^Accession: //' > Affymetrix_Genome-Wide_Human_SNP_Array.platform_accessions.txt
5. for platform in $(cat Affymetrix_Genome-Wide_Human_SNP_Array.platform_accessions.txt); do perl
~/scripts/esearch.pl gds $platform; done | sort -nu > Affymetrix_Genome-Wide_Human_SNP_Array.gds_ids.txt
6. paste cancer_copy_number.pubmed_ids.txt cancer_copy_number.pubmed_result.oneLine.txt | perl
~/scripts/table.addColumns.pl cancer_copy_number.pubmed_gds_ids.txt 0 - 0 1 | perl ~/scripts/table.search.pl
Affymetrix_Genome-Wide_Human_SNP_Array.gds_ids.txt 0 - 1 | perl ~/scripts/table.mergeLines.pl -d ', ' - 0,2 >
cancer_copy_number.Affymetrix_Genome-Wide_Human_SNP_Array.pubmed_gds.txt