Home / string-database / references/string_reference.md

references/string_reference.md

STRING Database API Reference

Overview

STRING (Search Tool for the Retrieval of Interacting Genes/Proteins) is a comprehensive database of known and predicted protein-protein interactions integrating data from over 40 sources.

Database Statistics (v12.0+): - Coverage: 5000+ genomes - Proteins: ~59.3 million - Interactions: 20+ billion - Data types: Physical interactions, functional associations, co-expression, co-occurrence, text-mining, databases

Core Data Resource: Designated by Global Biodata Coalition and ELIXIR

API Base URLs

  • Current version: https://string-db.org/api
  • Version-specific: https://version-12-0.string-db.org/api (for reproducibility)
  • API documentation: https://string-db.org/help/api/

Best Practices

  1. Identifier Mapping: Always map identifiers first using get_string_ids for faster subsequent queries
  2. Use STRING IDs: Prefer STRING identifiers (e.g., 9606.ENSP00000269305) over gene names
  3. Specify Species: For networks with >10 proteins, always specify species NCBI taxon ID
  4. Rate Limiting: Wait 1 second between API calls to avoid server overload
  5. Versioned URLs: Use version-specific URLs for reproducible research
  6. POST over GET: Use POST requests for large protein lists
  7. Caller Identity: Include caller_identity parameter for tracking (e.g., your application name)

API Methods

1. Identifier Mapping (get_string_ids)

Purpose: Maps common protein names, gene symbols, UniProt IDs, and other identifiers to STRING identifiers.

Endpoint: /api/tsv/get_string_ids

Parameters: - identifiers (required): Protein names/IDs separated by newlines (%0d) - species (required): NCBI taxon ID - limit: Number of matches per identifier (default: 1) - echo_query: Include query term in output (1 or 0) - caller_identity: Application identifier

Output Format: TSV with columns: - queryItem: Original query - queryIndex: Query position - stringId: STRING identifier - ncbiTaxonId: Species taxon ID - taxonName: Species name - preferredName: Preferred gene name - annotation: Protein description

Example:

identifiers=TP53%0dBRCA1&species=9606&limit=1

Use cases: - Converting gene symbols to STRING IDs - Validating protein identifiers - Finding canonical protein names

2. Network Data (network)

Purpose: Retrieves protein-protein interaction network data in tabular format.

Endpoint: /api/tsv/network

Parameters: - identifiers (required): Protein IDs separated by %0d - species: NCBI taxon ID - required_score: Confidence threshold 0-1000 (default: 400) - 150: low confidence - 400: medium confidence - 700: high confidence - 900: highest confidence - network_type: functional (default) or physical - add_nodes: Add N interacting proteins (0-10) - caller_identity: Application identifier

Output Format: TSV with columns: - stringId_A, stringId_B: Interacting proteins - preferredName_A, preferredName_B: Gene names - ncbiTaxonId: Species - score: Combined interaction score (0-1000) - nscore: Neighborhood score - fscore: Fusion score - pscore: Phylogenetic profile score - ascore: Coexpression score - escore: Experimental score - dscore: Database score - tscore: Text-mining score

Network Types: - Functional: All interaction evidence types (recommended for most analyses) - Physical: Only direct physical binding evidence

Example:

identifiers=9606.ENSP00000269305%0d9606.ENSP00000275493&required_score=700

3. Network Image (image/network)

Purpose: Generates visual network representation as PNG image.

Endpoint: /api/image/network

Parameters: - identifiers (required): Protein IDs separated by %0d - species: NCBI taxon ID - required_score: Confidence threshold 0-1000 - network_flavor: Visualization style - evidence: Show evidence types as colored lines - confidence: Show confidence as line thickness - actions: Show activating/inhibiting interactions - add_nodes: Add N interacting proteins (0-10) - caller_identity: Application identifier

Output: PNG image (binary data)

Image Specifications: - Format: PNG - Size: Automatically scaled based on network size - High-resolution option available (add ?highres=1)

Example:

identifiers=TP53%0dMDM2&species=9606&network_flavor=evidence

4. Interaction Partners (interaction_partners)

Purpose: Retrieves all STRING interaction partners for given protein(s).

Endpoint: /api/tsv/interaction_partners

Parameters: - identifiers (required): Protein IDs - species: NCBI taxon ID - required_score: Confidence threshold 0-1000 - limit: Maximum number of partners (default: 10) - caller_identity: Application identifier

Output Format: TSV with same columns as network method

Use cases: - Finding hub proteins - Expanding networks - Discovery of novel interactions

Example:

identifiers=TP53&species=9606&limit=20&required_score=700

5. Functional Enrichment (enrichment)

Purpose: Performs functional enrichment analysis for a set of proteins across multiple annotation databases.

Endpoint: /api/tsv/enrichment

Parameters: - identifiers (required): List of protein IDs - species (required): NCBI taxon ID - caller_identity: Application identifier

Enrichment Categories: - Gene Ontology: Biological Process, Molecular Function, Cellular Component - KEGG Pathways: Metabolic and signaling pathways - Pfam: Protein domains - InterPro: Protein families and domains - SMART: Domain architecture - UniProt Keywords: Curated functional keywords

Output Format: TSV with columns: - category: Annotation category - term: Term ID - description: Term description - number_of_genes: Genes in input with this term - number_of_genes_in_background: Total genes with this term - ncbiTaxonId: Species - inputGenes: Comma-separated gene list - preferredNames: Comma-separated gene names - p_value: Enrichment p-value (uncorrected) - fdr: False discovery rate (corrected p-value)

Statistical Method: Fisher's exact test with Benjamini-Hochberg FDR correction

Example:

identifiers=TP53%0dMDM2%0dATM%0dCHEK2&species=9606

6. PPI Enrichment (ppi_enrichment)

Purpose: Tests if a network has significantly more interactions than expected by chance.

Endpoint: /api/json/ppi_enrichment

Parameters: - identifiers (required): List of protein IDs - species: NCBI taxon ID - required_score: Confidence threshold - caller_identity: Application identifier

Output Format: JSON with fields: - number_of_nodes: Proteins in network - number_of_edges: Interactions observed - expected_number_of_edges: Expected interactions (random) - p_value: Statistical significance

Interpretation: - p-value < 0.05: Network is significantly enriched - Low p-value indicates proteins form functional module

Example:

identifiers=TP53%0dMDM2%0dATM%0dCHEK2&species=9606

7. Homology Scores (homology)

Purpose: Retrieves protein similarity/homology scores.

Endpoint: /api/tsv/homology

Parameters: - identifiers (required): Protein IDs - species: NCBI taxon ID - caller_identity: Application identifier

Output Format: TSV with homology scores between proteins

Use cases: - Identifying protein families - Paralog analysis - Cross-species comparisons

8. Version Information (version)

Purpose: Returns current STRING database version.

Endpoint: /api/tsv/version

Output: Version string (e.g., "12.0")

Common Species NCBI Taxon IDs

Organism Common Name Taxon ID
Homo sapiens Human 9606
Mus musculus Mouse 10090
Rattus norvegicus Rat 10116
Drosophila melanogaster Fruit fly 7227
Caenorhabditis elegans C. elegans 6239
Saccharomyces cerevisiae Yeast 4932
Arabidopsis thaliana Thale cress 3702
Escherichia coli K-12 E. coli 511145
Danio rerio Zebrafish 7955
Gallus gallus Chicken 9031

Full list: https://string-db.org/cgi/input?input_page_active_form=organisms

STRING Identifier Format

STRING uses Ensembl protein IDs with taxon prefix: - Format: {taxonId}.{ensemblProteinId} - Example: 9606.ENSP00000269305 (human TP53)

ID Components: - Taxon ID: NCBI taxonomy identifier - Protein ID: Usually Ensembl protein ID (ENSP...)

Interaction Confidence Scores

STRING provides combined confidence scores (0-1000) based on multiple evidence channels:

Evidence Channels

  1. Neighborhood (nscore): Gene fusion and conserved genomic neighborhood
  2. Fusion (fscore): Gene fusion events across species
  3. Phylogenetic Profile (pscore): Co-occurrence across species
  4. Coexpression (ascore): RNA expression correlation
  5. Experimental (escore): Biochemical/genetic experiments
  6. Database (dscore): Curated pathway/complex databases
  7. Text-mining (tscore): Literature co-occurrence
  • 150: Low confidence (exploratory analysis)
  • 400: Medium confidence (standard analysis)
  • 700: High confidence (conservative analysis)
  • 900: Highest confidence (very stringent)

Output Formats

Available Formats

  1. TSV: Tab-separated values (default, best for data processing)
  2. JSON: JavaScript Object Notation (structured data)
  3. XML: Extensible Markup Language
  4. PSI-MI: Proteomics Standards Initiative format
  5. PSI-MITAB: Tab-delimited PSI-MI format
  6. PNG: Image format (for network visualizations)
  7. SVG: Scalable vector graphics (for network visualizations)

Format Selection

Replace /tsv/ in URL with desired format: - /json/network - JSON format - /xml/network - XML format - /image/network - PNG image

Error Handling

HTTP Status Codes

  • 200 OK: Successful request
  • 400 Bad Request: Invalid parameters or syntax
  • 404 Not Found: Protein/species not found
  • 500 Internal Server Error: Server error

Common Errors

  1. "No proteins found": Invalid identifiers or species mismatch
  2. "Species required": Missing species parameter for large networks
  3. Empty results: No interactions above score threshold
  4. Timeout: Network too large, reduce protein count

Advanced Features

Bulk Network Upload

For complete proteome analysis: 1. Navigate to https://string-db.org/ 2. Select "Upload proteome" option 3. Upload FASTA file 4. STRING generates complete interaction network and predicts functions

Values/Ranks Enrichment API

For differential expression/proteomics data:

  1. Get API Key:
/api/json/get_api_key

  1. Submit Data: Tab-separated protein ID and value pairs

  2. Check Status:

/api/json/valuesranks_enrichment_status?job_id={id}
  1. Retrieve Results: Access enrichment tables and figures

Requirements: - Complete protein set (no filtering) - Numeric values for each protein - Proper species identifier

Network Customization

Network Size Control: - add_nodes=N: Adds N most connected proteins - limit: Controls partner retrieval

Confidence Filtering: - Adjust required_score based on analysis goals - Higher scores = fewer false positives, more false negatives

Network Type Selection: - functional: All evidence (recommended for pathway analysis) - physical: Direct binding only (recommended for structural studies)

Integration with Other Tools

Python Libraries

requests (recommended):

import requests
url = "https://string-db.org/api/tsv/network"
params = {"identifiers": "TP53", "species": 9606}
response = requests.get(url, params=params)

urllib (standard library):

import urllib.request
url = "https://string-db.org/api/tsv/network?identifiers=TP53&species=9606"
response = urllib.request.urlopen(url)

R Integration

STRINGdb Bioconductor package:

library(STRINGdb)
string_db <- STRINGdb$new(version="12", species=9606)

Cytoscape

STRING networks can be imported into Cytoscape for visualization and analysis: 1. Use stringApp plugin 2. Import TSV network data 3. Apply layouts and styling

Data License

STRING data is freely available under Creative Commons BY 4.0 license: - ✓ Free to use for academic and commercial purposes - ✓ Attribution required - ✓ Modifications allowed - ✓ Redistribution allowed

Citation: Szklarczyk et al. (latest publication)

Rate Limits and Usage

  • Rate limiting: No strict limit, but avoid rapid-fire requests
  • Recommendation: Wait 1 second between calls
  • Large datasets: Use bulk download from https://string-db.org/cgi/download
  • Proteome-scale: Use web upload feature instead of API
  • STRING website: https://string-db.org
  • Download page: https://string-db.org/cgi/download
  • Help center: https://string-db.org/help/
  • API documentation: https://string-db.org/help/api/
  • Publications: https://string-db.org/cgi/about

Troubleshooting

No results returned: - Verify species parameter matches identifiers - Check identifier format - Lower confidence threshold - Use identifier mapping first

Timeout errors: - Reduce number of input proteins - Split large queries into batches - Use bulk download for proteome-scale analyses

Version inconsistencies: - Use version-specific URLs - Check STRING version with /version endpoint - Update identifiers if using old IDs

← Back to string-database