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KEGG Overview

1. Genomes to Biological System

KEGG is a database resource for understanding high-level functions and utilities of the biological system, such as the cell, the organism and the ecosystem, from genomic and molecular-level information. It is a computer representation of the biological system, consisting of molecular building blocks of genes and proteins (genomic information) and chemical substances (chemical information) that are integrated with the knowledge on molecular wiring diagrams of interaction, reaction and relation networks (systems information). It also contains disease and drug information (health information) as perturbations to the biological system.

KEGG overview

The KEGG database has been in development by Kanehisa Laboratories since 1995, and is now a prominent reference knowledge base for integration and interpretation of large-scale molecular data sets generated by genome sequencing and other high-throughput experimental technologies.

2. The KEGG Database

KEGG is an integrated database resource consisting of sixteen databases shown below. They are broadly categorized into systems information, genomic information, chemical information and health information, which are distinguished by color coding of web pages.

Category Database Content Color
KEGG PATHWAY KEGG pathway maps kegg3
KEGG BRITE BRITE hierarchies and tables
KEGG ORTHOLOGY (KO) Functional orthologs kegg4
KEGG GENOME KEGG organisms (complete genomes) kegg1
KEGG GENES Genes and proteins
KEGG SSDB GENES sequence similarity
KEGG COMPOUND Small molecules kegg2
KEGG REACTION Biochemical reactions
KEGG RCLASS Reaction class
KEGG ENZYME Enzyme nomenclature
KEGG DISEASE Human diseases kegg5
KEGG DGROUP Drug groups
KEGG ENVIRON Health-related substances
Chemical information category is collectively called KEGG LIGAND
Health information category integrated with drug labels is called KEGG MEDICUS

These database contain various data objects for computer representation of the biological systems. Thus, the database entry of each database is called the KEGG object, which is identified by the KEGG object identifier consisting of a database-dependent prefix and a five-digit number (see: KEGG objects).

Release Database Object Identifier Remark
1995KEGG PATHWAYmap number
KEGG GENESlocus_tag / GeneID
2000KEGG GENOMEorganism code / T number
2002KEGG ORTHOLOGY  K numberOrtholog IDs in 2000
2003KEGG GLYCANG number
2004KEGG RPAIRRP numberDiscontinued in 2016
2005KEGG BRITEbr number
2006KEGG MODULEM number
2008KEGG DISEASEH number
2010KEGG ENVIRONE numberFirst called EDRUG
2014KEGG DGROUPDG number

3. KEGG Molecular Networks

The most unique data object in KEGG is the molecular networks -- molecular interaction, reaction and relation networks representing systemic functions of the cell and the organism. Experimental knowledge on such systemic functions is captured from literature and organized in the following forms:
  • Pathway map - in KEGG PATHWAY (see: Pathway maps)
  • Brite hierarchy and table - in KEGG BRITE (see: Brite hierarchies)
  • Membership (logical expression) - in KEGG MODULE
  • Membership (simple list) - in KEGG DISEASE
These databases constitute the reference knowledge base for biological interpretation of genomes and high-throughput molecular datasets through the process of KEGG mapping (see: KEGG mapping).

In 1995 the concept of mapping was first introduced in KEGG for linking genomes to metabolic pathways (metabolic reconstruction) using the EC number. Once the EC numbers were assigned to enzyme genes in the genome, organism-specific pathways could be generated automatically by matching against the enzyme (EC number) networks of the KEGG reference metabolic pathways. The EC number is no longer used as an identifier in KEGG. The KEGG Orthology (KO) system is the basis for genome annotation and KEGG mapping.

Period Identifier Reference knowledge Assignment
1995-1999 EC number Metabolic pathways Domain based
2000-2002 Ortholog ID Metabolic and regulatory pathways Domain based
2003- KO Pathways and BRITE hierarchies Gene based

From a different perspective, individual instances of genes are grouped into KO entries representing functional orthologs in the molecular networs. There are two more types of such generalization in KEGG as shown below.

Network type Class Instance
All types KO (gene ortholog) Genes in KEGG GENES
Biochemical reaction RC (reaction class) Reactions in KEGG REACTION
Drug interaction DG (drug group) Drugs in KEGG DRUG


  1. Kanehisa, M.; Toward pathway engineering: a new database of genetic and molecular pathways. Science & Technology Japan, No. 59, pp. 34-38 (1996). [pdf]
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  9. Kanehisa, M., Goto, S., Furumichi, M., Tanabe, M., and Hirakawa, M.; KEGG for representation and analysis of molecular networks involving diseases and drugs. Nucleic Acids Res. 38, D355-D360 (2010). [pubmed] [doi]
  10. Kanehisa, M., Goto, S., Sato, Y., Furumichi, M., and Tanabe, M.; KEGG for integration and interpretation of large-scale molecular datasets. Nucleic Acids Res. 40, D109-D114 (2012). [pubmed] [doi]
  11. Kanehisa, M., Goto, S., Sato, Y., Kawashima, M., Furumichi, M., and Tanabe, M.; Data, information, knowledge and principle: back to metabolism in KEGG. Nucleic Acids Res. 42, D199–D205 (2014). [pubmed] [doi]
  12. Kanehisa, M., Sato, Y., Kawashima, M., Furumichi, M., and Tanabe, M.; KEGG as a reference resource for gene and protein annotation. Nucleic Acids Res. 44, D457-D462 (2016). [pubmed] [doi]
  13. Kanehisa, Furumichi, M., Tanabe, M., Sato, Y., and Morishima, K.; KEGG: new perspectives on genomes, pathways, diseases and drugs. Nucleic Acids Res. 45, D353-D361 (2017). [pubmed] [doi]

Last updated: June 26, 2017