D512 Nucleic Acids Research, 2006, Vol. 34, Database issue Figure 1. A representative example of a pathway in MetaCyc. Pathways can be displayed at varying levels of detail. This pathway display depicts an intermediate level of detail including enzymes, EC numbers, genes and chemical structures of the main compounds. Notice the brown arrows that provide hyperlinks to related upstream and downstream pathways. For a thorough discussion of the major differences between MetaCyc and other pathway databases please see http:// metacyc.org/MetaCycUserGuide.shtml#otherpathwaydbs. DATA CONTENT 5 As demonstrated in Table 3, there has been a signiﬁcant increase in the number of database objects since the last Nucleic Acids Research publication 2 years ago (1). The num- ber of metabolic pathways has increased by 26% from 491 to 621, while the number of enzymes, genes and citations has 10 grown considerably more, by 75, 71 and 140%, respectively, owing to the fact that many existing pathways have been extensively edited and updated with comments, enzymes, genes and citations. There has also been a 128% increase in the number of organisms represented (currently at 506), 15 reﬂecting the breadth of MetaCyc (Tables 1 and 2), and a 57% increase in the number of chemical compounds (currently 4620). Data in MetaCyc are curated from the experimental liter- ature, which is read and summarized by PhD-level curators. Curators at SRI cover micr20obial and animal pathways, while curators at Carnegie Institution cover pathways from higher plants. MetaCyc contains the full complement of EcoCyc metabolic pathways (6) and thus has most of the basic path- ways of central and intermediary metabolism typical of enteric bacteria. Similarly, most o2f5 the pathways of central metabol- ism in higher plants are present in the database. Our current curation strategy focuses on both depth and breadth, by updat- ing pathways that lack complete information, and adding new pathways outside of central metabolism. The MetaCyc Curator’s 30Guide for Pathway/Genome Data- bases, located at URL http://bioinformatics.ai.sri.com/ptools/ curatorsguide.pdf, was developed to ensure the consistency of curation procedures. It documents the type of information that should be captured for each pathway, reaction, enzyme, gene and chemical compound. 3I5t also describes stylistic conven- tions. We recently revised it to explicitly deﬁne the organiza- tion of metabolic pathways in MetaCyc, and to provide