KEGG (Kyoto Encyclopedia of Genes and Genomes) is an integrated database resource consisting of 16 main databases, broadly categorized into systems information, genomic information and chemical information as shown in Table 1. Genomic and chemical information represents the molecular building blocks of life in the genomic and chemical spaces, respectively, and systems information represents functional aspects of the biological systems, such as the cell and the organism, that are built from the building blocks. The actual data contents of the genomic information category are: gene catalogs (KEGG GENES) in the completely sequenced genomes (KEGG GENOME), computationally derived sequence similarity relationships (KEGG SSDB), manually defined ortholog groups (KEGG ORTHOLOGY) and supplementary gene catalog data (KEGG DGENES and EGENES). The data contents of the chemical information category are: small molecules (KEGG COMPOUND), glycans (KEGG GLYCAN), reactions among them (KEGG REACTION), chemical structure transformation patterns derived from them (KEGG RPAIR) and supplementary information on enzyme nomenclature (KEGG ENZYME). The five databases in the chemical information category are collectively called KEGG LIGAND. Note that KEGG DRUG has been moved from the chemical information category
