• KEGG is a well-known pathway database, which provides reaction information (genome.jp/kegg/reaction), build on literature which is connected to their pathways (not directly to the reactions). This data is retrievable in an automated way through their Applicable Programming Interface (API), available at kegg.jp/kegg/rest/keggapi.html. The reaction information in KEGG is not directed, potentially causing biologically irrelevant paths when added to the DSMN. KEGG reactions provide a clear differentiation between side and main reactions by defining compound pairs according to a Reaction Classification pattern . Atom mappings are used to complete reaction equations if needed and to categorize the reactions in different subcategories. The reactions in KEGG are annotated with their own identifier type (starting with an R), and a link out to Rhea is provided on the website (not through the API). KEGG also uses its own identifier structure for metabolites (starting with a C for compounds, and a D for drugs); many of these are part of Wikidata and could therefore be added to the graph database as mapping identifiers. However, KEGG requires a license to work with their data; therefore we cannot include their data in the DSMN graph for redistribution. • MetaCyc , part of the BioCyc database collection, also allows for the retrieval of reactions in their pathways through an API (biocyc.org/web-services.shtml) and is built with an extensive literature background; even though this literature is depicted on the website in the pathway browser, the references are not directly connected to the reactions in the API. These reactions are directional, which can be deduced from the information provided in a ptools-xml format, where the substrate(s) are listed under "left" and the product(s) under "right". Since MetaCyc uses its own identifier structure (starting with "META:" and then an (abbreviated form of) the name of a compound), additional identifier mapping is needed to connect the information from this database to the network (which can be done by making an additional call to their API). The identifiers from MetaCyc are currently not part of Wikidata. No difference is made between side and main metabolic reactions, and reactions are annotated with their own identifier structure (for example "META:6PGLUCONOLACT-RXN"), with a link out to Rhea and KEGG. The addition of these identifiers can create ambiguity between identifiers describing the seemingly same reaction (since Rhea supports both directed and undirected reactions, while the latter only are listed in KEGG). Even though the data of MetaCyc is free to use, a license has to be requested to obtain the data, therefore not allowing this data to be distributed through the DSMN. • The Small Molecule Pathway DataBase (SMPDB, smpdb.ca) , provides pathways aimed at metabolite and drug interactions. The data can be downloaded in several formats, including BioPax and SBML, which are compatible with the output from WikiPathways after converting to a GPML model. However, the data format is not directly the same as the WikiPathways RDF, therefore the queries described previously have to be rewritten to obtain the reaction information from these formats. Directional reactions are provided, and several databases are used for the identification of compounds (e.g. HMDB, Drugbank, ChemSpider, PubChem). No difference is made between the main and side reaction(s) (participants). In order to build a comprehensive metabolic network extending the DSMN, identifier standardization to one database is required, which would mean an additional step before the data of SMPDB can be integrated. The data is free to use in non-commercial settings. • Pathway Commons is a database that aims to merge information from more than twenty public pathway-and interaction databases in one structured setting.