An integrated database resource consisting of 16 main databases, which are categorized into systems, genomic, chemical and health information. The PATHWAY, BRITE and MODULE databases in the systems information category contain KEGG pathway maps, BRITE hierarchy and table files and KEGG modules, respectively, as representations of high-level functions. They are all manually created based on published literature. The genomic information category contains the GENOME and GENES databases for collections of organisms with complete genomes and their gene catalogs, which are mostly taken from RefSeq and GenBank databases. The COMPOUND, GLYCAN, REACTION, RPAIR, RCLASS and ENZYME databases in the chemical information category contain chemical substances and reactions and are collectively called KEGG LIGAND for historical reasons. The health information category consists of the DISEASE, DRUG, DGROUP and ENVIRON databases for disease and drug information.