KEGG is a database resource that integrates genomic, chemical, and systemic functional information. It links gene catalogs from completely sequenced genomes to higher-level systemic functions of the cell, organism, and ecosystem. KEGG provides tools for mapping data to pathway, BRITE, and module maps, enabling integration and interpretation of large-scale molecular data sets. It also includes disease and drug information, allowing integration into molecular networks. KEGG Mapper is a collection of tools for KEGG pathway, BRITE, and module mapping, enabling the analysis of large-scale data sets. KEGG has been expanded to include disease and drug information, supporting translational bioinformatics. KEGG databases include 15 main databases covering various data objects, such as systems information, genomic information, and chemical information. KEGG GENES is a collection of gene catalogs for all organisms with completely sequenced genomes. KEGG pathway maps are graphical diagrams representing molecular interaction and reaction networks. BRITE functional hierarchies represent known functional hierarchies of genes, proteins, diseases, drugs, compounds, and reactions. KEGG modules are defined as tighter functional units than KEGG pathways. KEGG Orthology is a system for linking genes in the genome to nodes of the molecular network. Genome annotation in KEGG is cross-species annotation, finding orthologous genes in all available genomes for given K numbers. KEGG Mapper is the user interface for KEGG mapping, consisting of seven tools for mapping data to pathway, BRITE, and module maps. KEGG Mapper can be used to compare metabolic capabilities of different organisms, examine complementarity of host-symbiont, host-pathogen, and host-microbiome relationships, and examine collective features of pangenomes. KEGG DRUG is a comprehensive collection of approved drugs in Japan, USA, and Europe. KEGG DISEASE is an underdeveloped database aiming to computerize all human diseases with known genetic factors and all infectious diseases with known pathogen genomes. The mapping of disease and drug data from these databases is now incorporated in the daily KEGG update procedure. KEGG mapping approach is being further expanded to integrate disease and drug information used in practice and in society, as well as related data contained in outside resources. KEGG is made available at both the GenomeNet website and the KEGG website. The KEGG site will become the primary site handling all database update procedures. Funding for open access charge: Kyoto University Bioinformatics Center. Conflict of interest statement. None declared.KEGG is a database resource that integrates genomic, chemical, and systemic functional information. It links gene catalogs from completely sequenced genomes to higher-level systemic functions of the cell, organism, and ecosystem. KEGG provides tools for mapping data to pathway, BRITE, and module maps, enabling integration and interpretation of large-scale molecular data sets. It also includes disease and drug information, allowing integration into molecular networks. KEGG Mapper is a collection of tools for KEGG pathway, BRITE, and module mapping, enabling the analysis of large-scale data sets. KEGG has been expanded to include disease and drug information, supporting translational bioinformatics. KEGG databases include 15 main databases covering various data objects, such as systems information, genomic information, and chemical information. KEGG GENES is a collection of gene catalogs for all organisms with completely sequenced genomes. KEGG pathway maps are graphical diagrams representing molecular interaction and reaction networks. BRITE functional hierarchies represent known functional hierarchies of genes, proteins, diseases, drugs, compounds, and reactions. KEGG modules are defined as tighter functional units than KEGG pathways. KEGG Orthology is a system for linking genes in the genome to nodes of the molecular network. Genome annotation in KEGG is cross-species annotation, finding orthologous genes in all available genomes for given K numbers. KEGG Mapper is the user interface for KEGG mapping, consisting of seven tools for mapping data to pathway, BRITE, and module maps. KEGG Mapper can be used to compare metabolic capabilities of different organisms, examine complementarity of host-symbiont, host-pathogen, and host-microbiome relationships, and examine collective features of pangenomes. KEGG DRUG is a comprehensive collection of approved drugs in Japan, USA, and Europe. KEGG DISEASE is an underdeveloped database aiming to computerize all human diseases with known genetic factors and all infectious diseases with known pathogen genomes. The mapping of disease and drug data from these databases is now incorporated in the daily KEGG update procedure. KEGG mapping approach is being further expanded to integrate disease and drug information used in practice and in society, as well as related data contained in outside resources. KEGG is made available at both the GenomeNet website and the KEGG website. The KEGG site will become the primary site handling all database update procedures. Funding for open access charge: Kyoto University Bioinformatics Center. Conflict of interest statement. None declared.