As described above, a single BRENDA entry can lead to multiple valid mapped reactions if the trivial names were resolved to different SMILES strings. Although formally valid, not all reactions may necessarily be meaningful. For example, sugars were sometimes retrieved in their closed or opened form for the reactants and products respectively, as depicted in Fig. . We therefore counted the number of bond edits (and the number of involved atoms) and only kept the set of reactions with the minimal number of bond edits observed. For all mapped reactions in an EC class, we then computed the frequency of appearance of its corresponding reaction rule, and saved its relative frequency to the column 'quality'. This column can be used to filter the database for high-quality reactions. Often reactions with a low quality correspond to reactions that were balanced and atom mapped, but nevertheless erroneous. For example, within EC 1.1.1.1, the oxidation and reduction of hydroxy, oxy and carboxy groups occured frequently, as depicted in Fig. , whereas a rule showing an oxidation plus methyl shift only occured once, suggesting that the entry in BRENDA was possibly wrong. In fact, the depicted reaction was extracted from Ref. 66 which states that the reactant 3-methylbutanol was oxidized to 3-methylbutanal (not 3-methylbutanone as listed in BRENDA). The entry thus corresponds to a wrong extraction of products in BRENDA, which we found to occur frequently.