Database resources of the National Center
for Biotechnology
David L. Wheeler*, Deanna M. Church, Scott Federhen, Alex E. Lash,
Thomas L. Madden, Joan U. Pontius, Gregory D. Schuler, Lynn M. Schriml,
Edwin Sequeira, Tatiana A. Tatusova and Lukas Wagner
National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health,
Building 38A, 8600 Rockville Pike, Bethesda, MD 20894, USA
Received September 17, 2002; Accepted October 2, 2002
ABSTRACT
In addition to maintaining the GenBank(R) nucleic
acid sequence database, the National Center for
Biotechnology Information (NCBI) provides data
analysis and retrieval resources for the data in
GenBank and other biological data made available
through NCBI’s Web site. NCBI resources include
Entrez, PubMed, PubMed Central (PMC), LocusLink,
the NCBITaxonomy Browser, BLAST, BLAST Link
(BLink), Electronic PCR (e-PCR), Open Reading
Frame (ORF) Finder, References Sequence (RefSeq),
UniGene, HomoloGene, ProtEST, Database of Single
Nucleotide Polymorphisms (dbSNP), Human/Mouse
Homology Map, Cancer Chromosome Aberration
Project (CCAP), Entrez Genomes and related tools,
the Map Viewer, Model Maker (MM), Evidence Viewer
(EV), Clusters of Orthologous Groups (COGs) database,
Retroviral Genotyping Tools, SAGEmap, Gene
Expression Omnibus (GEO), Online Mendelian
Inheritance in Man (OMIM), the Molecular Modeling
Database (MMDB), the Conserved Domain Database
(CDD), and the Conserved Domain Architecture
Retrieval Tool (CDART). Augmenting many of the
Web applications are custom implementations of
the BLAST program optimized to search specialized
data sets. All of the resources can be accessed
through the NCBI home page at: http://
www.ncbi.nlm.nih.gov.
INTRODUCTION
The National Center for Biotechnology Information (NCBI) at
the National Institutes of Health was created in 1988 to
develop information systems for molecular biology. In addition
to maintaining the GenBank(R) (1) nucleic acid sequence
database, to which data is submitted by the scientific
community, NCBI provides data retrieval systems and
computational resources for the analysis of GenBank data
and a variety of other biological data. For the purposes of this
overview, the NCBI suite of database resources is grouped into
the six categories given below. All resources discussed are
available from the NCBI home page at: http://www.ncbi.nlm.
nih.gov. In most cases, the data underlying these resources is
available for bulk download at ‘ftp.ncbi.nih.gov’.
DATABASE RETRIEVAL TOOLS
Entrez
Entrez (2) is an integrated database retrieval system for DNA
and protein sequences derived from several sources (1,3–6),
the NCBI taxonomy, genome maps, population sets, gene
expression data, protein structures from the Molecular
Modeling Database (MMDB) (7), 3D and alignment-based
protein domains, and the biomedical literature via PubMed,
Online Mendelian Inheritance in Man (OMIM), and online
Books. PubMed includes primarily 12 million references and
abstracts in MEDLINE(R), with links to the full-text of more
than 3000 journals available on the Web. The Books database
now contains 12 online scientific textbooks.
Entrez enables text searching of databases ranging from
those for sequences and the scientific literature, to those for
structure and gene expression using simple Boolean queries,
and provides extensive links to related information. Some links
are simple cross-references, for example, from a sequence to
the abstract of the paper in which it was reported, from a
protein sequence to its corresponding DNA sequence or 3Dstructure,
or to alignments with other sequences. Other links
are based on computed similarities among the sequences or
MEDLINE abstracts. These pre-computed ‘neighbors’ allow
rapid access for browsing groups of related records. A service
called LinkOut expands the range of external links from
individual database records to related outside services,
including organism-specific genome databases.
The records retrieved by an Entrez search can be displayed in
a wide variety of formats and downloaded singly or in large
batches. Formatting options vary for records of different types.
*To whom correspondence should be addressed. Tel: þ1 3014962475/þ1 3014355950; Fax: þ1 3014809241; Email: wheeler@ncbi.nlm.nih.gov
28–33 Nucleic Acids Research, 2003, Vol. 31, No. 1 # 2003 Oxford University Press
DOI: 10.1093/nar/gkg033
For example, display formats for GenBank records include
the GenBank Flatfile, FASTA, XML, ASN.1, and others.
Graphical display formats are offered for some types of
records, including genomic records.
PubMed Central (PMC)
PMC is a digital archive of peer reviewed journals in the life
sciences. Over 90 journals, including Nucleic Acids Research,
now deposit the full text of their articles in PMC, which is
available at http://www.pubmedcentral.gov/. Participation in
PMC requires a commitment to free access to full text, perhaps
with some delay after publication. Some journals provide free
access to their full text directly in PMC while others require a
link to the journal’s own site where full text is generally
available free within 6 months to a year of publication. All
PMC free articles are identified in PubMed search results.
Taxonomy
The NCBI taxonomy database indexes over 119 000 organisms
that are represented in the databases with at least one
nucleotide or protein sequence. The Taxonomy Browser can
be used to view the taxonomic position or retrieve sequence
and structural data for a particular organism or group. Searches
of the NCBI taxonomy may be made on the basis of whole,
partial or phonetically spelled organism names, and links to
organisms commonly used in biological research are provided.
The Entrez Taxonomy system adds the ability to display
custom taxonomic trees representing user-defined subsets of
the full NCBI taxonomy.
NCBI is developing the non-bibliographic applications of
LinkOut, and expanding that project into the taxonomy and
sequence domains of Entrez. Many outside resources currently
maintain LinkOut links from Entrez entries, including model
organism and taxonomic databases. NCBI has developed
several tools to help LinkOut providers, including a simple
flatfile format for specifying links and a Name/ID status page
for tracking the current use of names and IDs in the taxonomy
database.
LocusLink
LocusLink, developed at NCBI in conjunction with several
international collaborators, offers a single query interface to
curated sequences and descriptive information about genes and
includes links to NCBI’s Map Viewer, Evidence Viewer (EV),
Model Maker (MM), BLAST Link (BLink), protein domains
from NCBI’s Conserved Domain Database, and many other
gene-related resources. LocusLink is discussed in a separate
article in this issue (6).
THE BLAST FAMILY OF SEQUENCE-SIMILARITY
SEARCH PROGRAMS
The Basic Local Alignment Search Tool (BLAST) programs
(8,9) perform sequence-similarity searches against a variety of
sequence databases, beginning with either a query sequence or
a GenBank accession number. BLAST returns a set of gapped
alignments between the query and similar database sequences,
with links to the full database records and to other relevant
databases such as UniGene or LocusLink. The sequences of
any or all of the database hits appearing in a BLAST alignment
may be selected for bulk download. A BLAST variant,
BLAST2Sequences (10), compares two DNA or protein
sequences using any of the standard BLAST programs and
produces a dot-plot representation of the alignments it reports.
Each alignment returned by a BLAST search receives a score
and a measure of statistical significance, called the Expectation
Value (E-value), for judging its quality. Either an E-value
threshold or a range can be specified to limit the alignments
returned. BLAST takes into account the amino acid composition
of the query sequence in its estimation of statistical
significance. This composition-based statistical treatment,
used in conventional protein BLAST searches as well as
PSI-BLAST (9) searches, tends to reduce the number of falsepositive
database hits (11).
The default BLAST output format is the ‘pairwise’
alignment, however several ‘query-anchored’ multiple
sequence alignment formats are available. An alignment
option, called the ‘Hit Table’, provides a compact, tabular,
easily parsable, summary of the BLAST results including, for
each database hit, the positions of alignment starts and stops,
scores, and Expectation Values. These outputs may be returned
in HTML, XML, text, or as ASN.1. In addition, BLAST can
generate a taxonomically organized output that shows the
distribution of BLAST hits by organism in three formats.
A particularly powerful feature of the web BLAST interface
allows searches to be restricted to a database subset using
standard Entrez search strings; the same restrictions may be
used to screen the output of an initially unrestricted search.
These features provide the means to effectively construct a
custom database for searching, or to process the output of a
search to include only sequences of interest. Web BLAST uses
a standard URL-API that allows complete search specifications,
including BLAST parameters, such as Entrez restrictions
and the search query, to be contained in a URL posted to the
web page.
A recent addition to the BLAST family, called MegaBLAST
(12), facilitates batch nucleotide queries which can be pasted
into a web page or uploaded from a file. MegaBLAST is
designed to search for nearly exact matches and is up to 10
times faster than standard BLAST for such searches.
MegaBLAST is provided to search entire eukaryotic genomes,
but it is also used to search a rapidly growing database, called
the Trace Archive, which contains over 125 million sequencing
traces. The Trace Archive includes whole genome shotgun
(WGS), shotgun, EST, clone end and finishing reads from
over 30 organisms such as Homo sapiens, Mus musculus,
Rattus norvegicus, Danio rerio, Zea mays and Caenorhabditis
elegans. To facilitate rapid cross-species nucleotide queries of
the Trace Archive, NCBI offers a version of MegaBLAST
called Discontinuous MegaBLAST that uses a non-contiguous
word match (13) as the nucleus for its alignments. Searches
using Discontinuous MegaBLAST are far more rapid than
cross-species translated searches such as blastx, but maintain a
competitive degree of sensitivity when comparing coding
regions.
The NCBI-generated assembly of the human as well as other
submitted genomic assemblies, such as those of the mouse and
zebrafish, may be searched using specialized genome BLAST
Nucleic Acids Research, 2003, Vol. 31, No. 1 29
pages. These pages search a set of genome-specific databases
and generate, where possible, genomic views of the BLAST
hits using the Map Viewer.
BLink
BLink displays pre-computed protein BLAST alignments for
each protein sequence in the Entrez databases. BLink allows
for the display of subsets of these alignments by taxonomic
criteria, by database of origin, relation to a complete genome,
membership in a Clusters of Orthologous Group (COG) (14) or
by relation to a 3D structure or conserved protein domain.
BLink links are displayed for protein records in Entrez as well
as within LocusLink reports.
RESOURCES FOR GENE-LEVEL SEQUENCES
UniGene
UniGene (15), is a system for automatically partitioning
GenBank sequences, including ESTs, into a non-redundant set
of gene-oriented clusters. UniGene clusters ESTs from 10
animals and 7 plants, bringing the total number of organisms
represented to 17. UniGene starts with entries in the
appropriate organism division of GenBank, combines these
with ESTs of that organism and creates clusters of sequences
that share virtually identical 30
untranslated regions (30
UTRs).
Each UniGene cluster contains sequences that represent a
unique gene, and is linked to related information, such as the
tissue types in which the gene is expressed, model organism
protein similarities, the LocusLink report for the gene and its
map location. In the human UniGene database, over 3.6
million human ESTs in GenBank have been reduced 35-fold in
number to over 104 000 sequence clusters. The UniGene
collection has been used as a source of unique sequence for
the fabrication of microarrays for the large-scale study of gene
expression (16). UniGene databases are updated weekly with
new EST sequences, and bimonthly with newly characterized
sequences. UniGene clusters may be searched in several ways;
by gene name, chromosomal location, cDNA library, accession
number, and ordinary text words. Cluster sequences may also
be downloaded by FTP.
ProtEST
ProtEST, a tool analogous to BLink, presents pre-computed
BLAST alignments between protein sequences from model
organisms and the 6-frame translations of UniGene nucleotide
sequences. Protein sequences that are derived from conceptual
translations or model transcripts are excluded. The eight model
organisms included are H. sapiens, M. musculus, Rattus
norvegicus, Drosophila melanogaster, C. elegans, Saccharomyces
cerevisiae, Arabidopsis thaliana and Escherichia coli.
ProtEST links are displayed in UniGene reports with model
organism protein similarites. For each nucleotide sequence
match, the ProtEST report shows the UniGene cluster ID, the
GenBank accession number, and the percent identity between
the protein and nucleotide translation in the aligned region. A
link is also provided to the sequence trace in the NCBI Trace
Archive, if available. ProtEST reports are updated in tandem
with UniGene protein similarities.
HomoloGene
HomoloGene is a database of both curated and calculated gene
orthologs and homologs for 14 organisms including H. sapiens,
M. musculus, D. rerio, D. melanogaster, C. elegans, A. thaliana,
Hordeum vulgare, Oryza sativa, Z. mays. Curated orthologs
include gene pairs from the Mouse Genome Database (MGD) at
the Jackson Laboratory, the Zebrafish Information (ZFIN)
database at the University of Oregon and from published
reports. Computed orthologs and homologs, which are
considered putative, are identified from BLAST nucleotide
sequence comparisons between all UniGene clusters for each
pair of organisms. HomoloGene also contains a set of triplet
ortholog-based COG (14)-like clusters, which may include up to
14 members, in which the triplet orthologs in two organisms are
both orthologous to the same gene in a third organism. For the
three organisms human, mouse and rat, there are currently
over 7000 of these self-consistent triplets. The HomoloGene
database can be queried using UniGene ClusterIDs, LocusLink
LocusIDs, gene symbols, gene names and nucleotide accession
numbers, as well as those terms found in UniGene cluster
titles. The current datasets for the calculated orthologs and
homologs and the Mutually Orthologous Pairs are also
available via FTP.
References Sequence (RefSeq)
The RefSeq database (6), provides curated reference sequences
for mRNAs, genomic sequences, computationally-derived
sequences and proteins for human and over 1700 other
organisms.
Open Reading Frame (ORF) Finder
ORF Finder performs a six-frame translation of a nucleotide
sequence and returns a graphic that indicates the location of
each ORF found. Restrictions on the size of the ORFs returned
may be set. The protein translations of the ORFs detected can
be submitted directly for BLAST similarity searching or
searching against the COGs (see below) database.
Electronic PCR (e-PCR)
e-PCR is a tool for locating Sequence Tagged Sites (STSs)
within a nucleotide sequence by searching against a nonredundant
database of over 133 000 human and 84 000 nonhuman
STSs called UniSTS.
A database of Single Nucleotide Polymorphisms (dbSNP)
The dbSNP (17) is a repository for single base nucleotide
substitutions and short deletion and insertion polymorphisms.
The dbSNP database contains almost 3 million human SNPs
as well as about half a million from organisms including
M. musculus, Anopheles gambiae, D. rerio and A. thaliana.
The Web interface allows flexible searches by gene name and
by cross-reference to other databases such as OMIM or the
structure databases. Searches for SNPs lying between two
markers and batch downloads are also supported. SNP reports
link to structures from the MMDB, allowing the 3-D
visualization, using NCBI’s interactive macromolecular viewer
30 Nucleic Acids Research, 2003, Vol. 31, No. 1
Cn3D (18), of amino acid changes implied by SNPs in coding
regions.
RESOURCES FOR GENOME-SCALE ANALYSIS
Entrez Genomes
Entrez Genomes (19) provides access to genomic data
contributed by the scientific community for over 1000 species
whose sequencing and mapping is complete or in progress.
Entrez Genomes now includes more than 86 complete
microbial genomes and 302 RefSeq for eukaryotic organelles.
Many higher eukaryotic genomes are also included within
Entrez Genomes such as those of H. sapiens, M. musculus,
D. melanogaster, Anopheles gambiae, C. elegans and A.
thaliana.
In Entrez Genomes, complete genomes can be accessed
hierarchically starting from either an alphabetical listing or a
phylogenetic tree for each of six principle taxonomic groups.
One can follow the hierarchy to a graphical overview for the
genome of a single organism, on to the level of a single
chromosome and, finally, down to the level of a single gene. At
each level are one or more views, pre-computed summaries,
and links to analyses appropriate at that level. For instance, at
the level of a genome or a chromosome, a coding regions view
displays the location of each coding region, length of the
product, GenBank identification number for the protein
sequence and name of the protein product. A RNA genes
view lists the location and gene names for ribosomal and
transfer RNA genes. At the level of a single gene, links are
provided to pre-computed sequence neighbors for the gene
product. Any protein gene product that is a member of a
COG (19) is linked to the COGs database. A summary of COG
functional groups is also presented in tabular and graphical
formats at the genome level.
For complete microbial genomes, pre-computed BLAST
neighbors for protein sequences, including their taxonomic
distribution and links to 3-D structures, are given in TaxTables
and PDBTables, respectively. Pairwise sequence alignments
are presented graphically and linked to the Cn3D macromolecular
viewer (18), which allows the interactive display of
3-D structures and sequence alignments. The TaxPlot tool,
graphically compares similarities in the proteomes of two
organisms to that of a third, reference organism and is available
for both prokaryotic and eukaryotic genomes. Resources for
the genomes of higher eukaryotes are discussed below.
COGs
The COGs database (14), presents a compilation of orthologous
groups of proteins from completely sequenced organisms
representing 44 species and 30 phylogenetically distant clades.
The COGs are now also linked to the proteins of two higher
eukaryotes; C. elegans and D. melanogaster.
Retroviral genotyping tools
The genotyping of retrovirus sequences is important in the
characterization of viral genetic diversity, in the tracking of
epidemics, and in vaccine development. NCBI offers a Webbased
genotyping tool that employs a blastn comparison
between a retroviral sequence to be subtyped and a default
panel of reference sequences or a panel provided by the user.
An HIV-1-specific subtyping tool uses a set of reference
sequences taken from the principle HIV-1 variants.
Eukaryotic Genomic Resources
Entrez Genomes links to Genome Resources webpages devoted
to the sequencing of a number of eukaryotic organisms
including H. sapiens, M. musculus and D. melanogaster. A
page called Plant Genomes Central serves as a collection point
for resources related to plant genome projects. Many genome
projects have progressed to the point at which it is useful to have
an interactive genome viewing tool with which to correlate the
data present in various of genomic maps. NCBI has developed
the Map Viewer for this purpose.
Map Viewer
The NCBI Map Viewer displays genome assemblies using sets
of synchronized chromosomal maps. Map Viewer displays are
available for the genomes of four vertebrates, including
H. sapiens and M. musculus and D. rerio, three invertebrates,
including D. melanogaster and C. elegans, seven plants,
including A. thaliana and O. satvia, and two fungi, S. cerevisiae
and Schizosaccharomyces pombe. The genomic maps displayed
by the Map Viewer vary according to the data available
for the subject organism. The maps can be selected from a set
of cytogenetic maps, such as chromosomal ideograms,
sequence-based maps, such as those showing contigs, genes,
and SNPs, and physical maps, such as the G3 and GB4 human
radiation-hybrid maps. Maps showing ab initio gene models,
EST alignments with links to UniGene clusters, and mRNA
alignments used to construct gene models are also available for
some organisms. The rightmost map in a Map Viewer display,
called the master map, generates an extended set of mapspecific
links to related resources. In the case of the Genes
map, two of these links are to the EVand MM described below.
In addition to its graphical display, the Map Viewer offers a
tabular view of the data that is convenient for export to other
programs for further analysis.
Queries against an entire genome or particular chromosomes
can be made in the Map Viewer using gene names or symbols,
marker names, SNP identifiers, accession numbers and other
identifiers. The human version of the Map Viewer is tightly
integrated with other NCBI databases such as LocusLink and
dbSNP. Segments of a genomic assembly may be downloaded
using the Map Viewer’s ‘Download/View Sequence’ link for
some genomes such as H. sapiens, M. musculus and A. thaliana.
Supported download formats are GenBank and FASTA.
Model Maker (MM)
MM allows the construction of transcript models using novel
combinations of putative exons derived from ab initio
predictions or from the alignment of GenBank transcripts,
including ESTs, and NCBI RefSeqs, to the NCBI human
genome assembly. The MM interface consists of a graphical
overview of transcript alignments to a genomic contig with
each unique block of alignment collected and numbered as a
putative exon. Transcript models are constructed by selecting
Nucleic Acids Research, 2003, Vol. 31, No. 1 31
from this collection. As the transcript is created, the implied
protein translation is given in each reading frame with any
internal stop codons indicated. Previously, observed exon
splice patterns are indicated as guides to model building.
Completed models may be saved locally or analyzed with
OrfFinder.
Evidence Viewer (EV)
The EV displays the alignments to a genomic contig of RefSeq
transcripts, GenBank mRNAs, known or potential transcripts,
and ESTs supporting a gene model. The EV produces a
graphical summary of the alignments that indicates the
coordinate range of the gene model on the genomic contig
and the areas of alignment to the transcripts on separate tracks.
EST alignment density along the contig is indicated on another
track. A mismatch and an insertion/deletion track are also
shown to highlight areas of disagreement between transcript
sequences and the genomic sequence. Following the graphical
summary are exon-by-exon alignments of all of the transcript
sequences against the genomic contig, including flanking
genomic sequence for each exon to show the presence or
absence of splice sites. Any proteins annotated on the
transcript sequences are also shown and mismatches between
transcripts and the genomic contig or between proteins
annotated on the aligned transcripts are highlighted.
The Human–Mouse Homology Maps
The Human–Mouse Homology Maps are tables of genetic loci
in homologous segments of DNA from human and the mouse.
The map is computed by integrating orthologs curated by the
Mouse Genome Database with putative orthologs identified by
homology. The maps are linked to GeneMap’99, OMIM,
LocusLink, dbSTS, BLAST2Sequences and the Mouse
Genome Database at The Jackson Laboratory. Other mouse
genome resources can be found on the Mouse Genome
Resources page.
The Cancer Chromosome Aberration Project (CCAP)
The CCAP service is an initiative of the National Cancer
Institute (NCI) and NCBI. The data includes a compilation by
F. Mitelman, F. Mertens and B. Johansson of recurrent
neoplasia-associated chromosomal aberrations from the
Cancer Chromosome Aberration Bank at the University of
Lund, Sweden (20). The Spectral Karyotyping database, SKY,
created jointly by NCI and NCBI, enables investigators to share
their own SKYand Comparative Genomic Hybidization (CGH)
data on chromsomal aberrations (http://www.ncbi.nlm.nih.gov/
sky/skyweb.cgi).
RESOURCES FOR THE ANALYSIS OF PATTERNS
OF GENE EXPRESSION AND PHENOTYPES
SAGEmap
Serial Analysis of Gene Expression (SAGE) is a technique for
taking a snapshot of the messenger RNA population of a cell
to obtain a quantitative measure of gene expression. NCBI’s
SAGEmap (21) service implements many functions useful in
the analysis of SAGE data such as a two-way mapping between
SAGE tag and UniGene. SAGEmap can also construct a userconfigurable
table of data comparing one group of SAGE
libraries with another. Groups may be chosen for inclusion in
the table on the basis of several expression criteria. SAGEmap
is updated weekly, immediately following the update of
UniGene and the data is reflected in the human genome Map
Viewer as the SAGE track.
Gene Expression Omnibus (GEO)
The GEO (22) is a data repository and retrieval system for gene
expression data derived from any organism or artificial source.
Gene expression data derived from spotted microarray, highdensity
oligonucleotide array, hybridization filter and SAGE
data, are available for download and accepted for deposit. At
the time of writing, the repository contains high-throughput
gene expression data on over 2300 samples.
OMIM
NCBI provides the online version of the OMIM catalog of
human genes and genetic disorders authored and edited by
Victor A. McKusick at The Johns Hopkins University (23).
The database contains information on disease phenotypes and
genes, including extensive descriptions, gene names, inheritance
patterns, map locations and gene polymorphisms. OMIM
currently contains 13 864 entries, including data on 10 290
established gene loci and 1019 phenotypic descriptions, and is
now searchable using the powerful Entrez interface.
THE MOLECULAR MODELING DATABASE
(MMDB), THE CONSERVED DOMAIN DATABASE
SEARCH AND CDART
The NCBI MMDB, built by processing entries from the Protein
Data Bank (5), is described in (7). The structures in the MMDB
are linked to sequences in Entrez and to the Conserved Domain
Database (CDD). The CDD contains PSI-BLAST-derived
Position Specific Score Matrices representing domains taken
principally from two public protein domain collections, the
Simple Modular Architecture Research Tool (SMART) (24),
and Pfam (25), but also draws from domains defined by NCBI
researchers. NCBI’s Conserved Domain Search (CD-Search)
service can be used to search a protein sequence for conserved
domains in the CDD. Wherever possible CDD hits are linked
to structures which, coupled with a multiple sequence alignment
of representatives of the domain hit, can be viewed with
NCBI’s 3-D molecular structure viewer, Cn3D (18). The
Conserved Domain Architecture Retrieval Tool (CDART)
allows searches of protein databases on the basis of a conserved
domain and returns the domain architectures of database
proteins conatining the query domain. Alignment-based protein
domain information from the CDD and 3-D domains from the
MMDB are searchable via the Entrez interface.
FOR FURTHER INFORMATION
Most of the resources described here include documentation,
other explanatory material and references to collaborators and
32 Nucleic Acids Research, 2003, Vol. 31, No. 1
data sources on the respective web sites. Several tutorials are
also offered under the Education link from NCBI’s home page.
A site map provides a comprehensive table of NCBI resources,
and the about NCBI feature provides bioinformatics primers
and other supplementary information. A user support staff is
available to answer questions at info@ncbi.nlm.nih.gov.
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