D758 Nucleic Acids Research, 2012, Vol. 40, Database issue Table 2. Resources relating to biochemical reactions Resource BioCyc: MetaCyc (10) and EcoCyc (13) Biopath (21) BKM-react (26) BRENDA (12) ENZYME (3) ExplorEnz (2) EzCatDB (19) IntEnz (17) KEGG (11) MACiE (16) Reactome (15) SABIO-RK (20) UniPathway (14) Location MetaCyc (10) http://metacyc.org; EcoCyc (13) http://ecocyc.org http://www.molecular-networks.com/biopath/ http://bkm.tu-bs.de/ www.brenda-enzymes.org http://enzyme.expasy.org/ http://www.enzyme-database.org http://mbs.cbrc.jp/EzCatDB/ http://www.ebi.ac.uk/intenz http://www.genome.jp/kegg/ http://www.ebi.ac.uk/thornton-srv/databases/MACiE/ http://www.reactome.org/ http://sabio.villa-bosch.de/ http://www.unipathway.org/pathway Figure 3. Sample master reaction and associated directional reactions in Rhea. http://www.ebi.ac.uk/rhea/reaction.xhtml?id=15133 RHEA:15133 is a master reaction. This master reaction is described by its chemicals (labels and identiﬁers in ChEBI), and has no speciﬁc direction. The three associated directional reactions are indicated in the section titled ‘Same participants, different directions’. These are: RHEA:15134 (left-to-right), RHEA:15135 (right-to-left) and RHEA:15136 (bidirectional). The section titled ‘Cross-references’ lists all related resources to which one of the directional reactions has been linked. The actual Rhea reaction can be determined from the directional icon (‘=>’ for RHEA:15134, ‘<=’ for RHEA:15135 and ‘<=>’ for RHEA:15136). The user can retrieve a list of all the reactions a speciﬁc compound is involved in by clicking the binocular symbol next to the compound name. Marvin is used for displaying chemical structures, Marvin 5.0.0, 2011 (http://www.chemaxon.com). Rhea web server The Rhea web server (http://www.ebi.ac.uk/rhea) enables access to Rhea data. It provides browsing, searching, web services and download facilities. An example of a reaction page is shown in Figure 3. It presents the chemical transformation (including the reaction participants’ names, ChEBI identiﬁers and chemical structures), and related information (cross-references and citations). The Rhea web site provides a number of search facilities (described below), and also allows users to navigate the reaction set via their common compounds. The Rhea website allows simple queries using any of the following as input: . a reaction identiﬁer from Rhea or any of the cross- referenced resources [KEGG (11), EcoCyc (13),