The systems information category is the most unique feature in the KEGG resource. Data and knowledge about the molecular systems that govern cellular processes and organism behaviors are manually collected and summarized from literature and presented in computable forms, called ‘pathway map’ (graph), ‘simple list’ (membership) and ‘hierarchical list’ (ontology), as illustrated in Figure 1. The pathway map in KEGG PATHWAY represents our knowledge about various types of molecular networks: reaction/interaction networks for metabolism, genetic information processing, environmental information processing and other cellular processes; perturbed reaction/interaction networks for human diseases; and relation networks (chemical structure transformation networks) for drug development. The molecular network shown in the pathway map is a graph consisting of nodes (orthologs, genes, proteins, small molecules, etc.) and edges (reactions, interactions and relations). In contrast, the simple list is a set of nodes without the wiring information, simply representing the membership to a molecular system. This representation is used when the detail of the molecular system is not known (KEGG DISEASE) or when a tighter functional unit is indicated within the molecular network (KEGG MODULE). The hierarchical
