STRING: known and predicted protein–protein
associations, integrated and transferred across
Christian von Mering, Lars J. Jensen, Berend Snel1, Sean D. Hooper, Markus Krupp,
Mathilde Foglierini, Nelly Jouffre, Martijn A. Huynen1and Peer Bork*
European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany and1Nijmegen Centre for
Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
Received September 11, 2004; Accepted September 13, 2004
A full description of a protein’s function requires
knowledge of all partner proteins with which it speci-
fically associates. From a functional perspective,
‘association’ can mean direct physical binding, but
can also mean indirect interaction such as partici-
pation in the same metabolic pathway or cellular
association is scattered over a wide variety of
resources and model organisms. STRING aims to
simplify access to this information by providing a
comprehensive, yet quality-controlled collection of
protein–protein associations for a large number of
organisms. The associations are derived from high-
throughput experimental data, from the mining
of databases and literature, and from predictions
based on genomic context analysis. STRING inte-
grates and ranks these associations by bench-
marking them against a common reference set,
and presents evidence in a consistent and intuitive
web interface. Importantly, the associations are
extended beyond the organism in which they were
originally described, by automatic transfer to ortho-
able. STRING currently holds 730000 proteins in
180 fully sequenced organisms, and is available at
Several databases exist, whose main purpose is to collect and
curate direct experimental evidence about protein–protein
interactions (1–4). Other databases take a more generalized
perspective on proteins and their associations, by functionally
grouping proteins into metabolic, signaling or transcriptional
pathways (5–8). Finally, a third class of resources attempts
to fill gaps in both datasets, by predicting protein–protein
associations de novo, using a variety of computational tech-
The database STRING (‘Search Tool for the Retrieval of
Interacting Genes/Proteins’) represents an ongoing effort to
provide these three types of protein–protein association evi-
dence under one common framework. Such an integrated
approach offers several unique advantages: (i) various types
of evidence are mapped onto a single, stable set of proteins,
thereby facilitating comparative analysis; (ii) known and
predicted interactions often partially complement each other,
leading to increased coverage; (iii) an integrated scoring
scheme can provide higher confidence when independent
evidence types agree; and (iv) mapping and transferring
interactions onto a large number of organisms facilitates
BecauseSTRING isfullypre-computed, allinformationcan
be quickly accessed—both at the high-level network view and
at the level of the individual interaction record. The various
evidence types can be enabled or disabled separately, which
allows the searches to be customized at run-time, and dedi-
cated viewers allow the inspection of all the evidence under-
lying an association (Figure 1). The database is an exploratory
resource: it contains a much larger number of associations
than primary interaction databases—albeit with varying
confidence scores. It is thus best used for getting a quick
initial overview of the functional partners of a query protein,
especially for proteins that are still poorly characterized.
DATA SOURCES AND SCORING
Many of the protein–protein associations in STRING are
imported from other databases (see below), but STRING
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Nucleic Acids Research, Vol. 33, Database issue ª Oxford University Press 2005; all rights reserved
Nucleic Acids Research, 2005, Vol. 33, Database issueD433–D437
also contains a large body of predicted associations that
are produced de novo. These predictions are based on system-
atic genome comparisons [‘genomic context’, (14,15)].
We periodically import completely sequenced genomes
[metazoan genomes from Ensembl, all others from SwissProt,
(16)], and search them for three types of genomic context
associations: conserved genomic neighborhood, gene fusion
events, and co-occurrence of genes across genomes. All three
searches aim to identify pairs of genes which appear to be
under common selective pressures during evolution (more so
than expected by chance), and which are therefore thought to
be functionally associated.
As for all other types of associations in STRING, we
assign a confidence score to each predicted association.
The scores are derived by benchmarking the performance
of the predictions against a common reference set of trusted,
true associations. We chose the functional grouping of
proteins maintained at KEGG [Kyoto Encyclopedia of
Genes and Genomes, (5)] as the reference. Any predicted
association for which both proteins are assigned to the
same ‘KEGG pathway’ is counted as a true positive.
KEGG pathways are particularly suitable as a reference
because they are based on manual curation, are available
for a number of organisms, and cover several functional
areas. The benchmarked confidence scores in STRING
generally correspond to the probability of finding the linked
proteins within the same KEGG pathway. STRING performs
a similar benchmark for high-throughput experimental inter-
action data, separately for each dataset. Scores vary within
one dataset because they include additional, intrinsic infor-
mation from the data itself, such as the frequency or recipro-
cality of the detection (see Figure 2 for a typical benchmark).
In contrast to high-throughput data, validated small-scale
interactions, protein complexes, and annotated pathways
Figure1.Resultsfroma STRINGsearch.Insertsshowpartialscreenshotsfromevidencepages,which areaccessiblefromthemainresultpage.Two proteinswere
used as inputs to the query—one is a subunit from the yeast ATP synthase complex, the other a subunit from the ubiquinol–cytochrome C reductase complex. The
number of requested partners was limited to 10 (default settings). STRING reports both proteins to be members of functional modules, which are in turn connected
as part of a larger unit. The diversity of evidence types supporting the modules is noted.
D434 Nucleic Acids Research, 2005, Vol. 33, Database issue
are directly imported from databases (2,5,17), and given
a uniform confidence score per dataset.
Another important sourceof protein associationinformation
is the published literature (18,19). We systematically extract
associations from PubMed, by searching for recurrent co-
mentioning of gene names in abstracts. This search relies
on gene names and synonyms parsed from SwissProt as
well as from organism-specific databases, and we utilize a
benchmarked scoring system based on the frequencies and
distributions of gene names in abstracts (not shown).
Finally, we also derive protein–protein associations from
functional genomics data: co-regulation of genes across
diverse experimental conditions, as measured by using micro-
array analysis, can be a predictor of functional associations
(20). We import these associations from the ArrayProspector
server (12), which is based on the same benchmarks and
genomes as STRING itself.
TRANSFER OF ASSOCIATIONS ACROSS
STRING employs two different strategies for transferring
(Figure 3): the first (‘COG-mode’) relies on externally pro-
vided orthology assignments and transfers interactions in an
all-or-none fashion, whereas the second (‘protein-mode’)
uses quantitative sequence similarity searches and often
distributes a given interaction fractionally among several
protein pairs of the target organism. Both approaches
have strengths and weaknesses, and users can choose
either one of them before starting their query (a color change
helps them to distinguish the modus throughout the user
The COG mode requires an assignment of proteins
group are assumed to be functionally equivalent across
genomes. This orthology information is imported from the
COGs database [(21), we extend the groups to cover all
organisms in STRING]. Any association score observed
between a pair of proteins from two different COGs is
assumed to be valid for all protein pairs spanning these
two COGs. Repeated observations of links, e.g. occurrence
of genes in the same operon, increase the association
score—but only when they are observed in phylogenetically
In the newly developed protein mode, there is no preas-
signed orthology information. Instead, the transfer relies
on a precomputed all-against-all similarity search of the
730000 proteins in STRING (using the sensitive Smith-
Waterman algorithm). For each association to be transferred,
the algorithm searches for potential orthologs of the inter-
acting partners in other genomes. Orthology is assumed if
proteins form reciprocal best matches in the searches, in the
absence of any close, second-best hits (paralogs) in either
species. In such an ideal situation, the interactions can be
transferred in toto. However, in reality there will often be
additional paralogs in one or both of the genomes, which
complicates the transfer. We have devised and benchmarked
an empirical scheme that is based on the relative sequence
similarity of competing paralogous proteins (Figure 3).
Essentially, the pair of proteins exhibiting the highest
sequence similarity to the source pair receives the highest
‘share’ of the transferred interaction.
After assignment of association scores and transfer between
species, we compute a final ‘combined score’ between any
pair of proteins (or pair of COGs). This score is often higher
than the individual sub-scores, expressing increased confi-
dence when an association is supported by several types of
evidence (Table 1). It is computed under the assumption of
independence for the various sources, in a naı ¨ve Bayesian
fashion. It is thus a simple expression of the individual
The assumption of independence is valid here because
datasets that are based on similar technologies (e.g.
different yeast two-hybrid datasets) have been joined pre-
viously and are benchmarked as a single information
source. Along with the combined score, the individual
sub-scores are always displayed as well, because they pro-
vide valuable information about the nature of a particular
S = 1 ?
1 ? Si
Figure 2. Deriving confidence scores for high-throughput interaction data
[exemplified here for a dataset of protein complex purifications (22)]. In
this case, the relative confidence depends on how often two proteins are
pulled down together (a and b), versus how often they are pulled down
alone (c and d). A purification is counted twice when one of the partners is
the bait (a and d). Raw quality is: Q = logf(Ntogether? Ntotal)/[(Nalone1+ 1) ?
Nucleic Acids Research, 2005, Vol. 33, Database issueD435
This work was supported in part by grants from the
Bundesministerium f€ u ur Forschung und Bildung, Germany,
from the Netherlands Organization of Scientific Research
(NOW),and fromThe Knut
Foundation (to S.D.H.).
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