 In this tutorial, we will learn how to search for Rhea reactions by enzyme class or EC number and how reactions in Rhea are mapped to EC numbers. The Nomenclature Committee of the International Union of Biochemistry and Molecular Biology maintains a numerical classification scheme that classifies enzymes according to their reactions they catalyze. In this classification enzymes are associated with a code called EC number which consists of the letters EC followed by four numbers separated by periods. Those numbers represent a progressively finer classification of the enzymatic activity. An EC number where all four positions are defined is commonly called complete EC number. In partial EC numbers the last number is replaced by a dash. Rhea reactions are mapped to complete EC numbers. As of the current Rhea release, more than 90% of all EC numbers are mapped to a Rhea reaction and our biocurators aim at keeping Rhea in sync with the enzyme classification. On the other hand, as Rhea describes more enzyme reactions compared to EC numbers, just over half of the Rhea reactions can be mapped to EC numbers. Some spontaneous reactions, for example, will never get an EC number. Making links in both directions is an ongoing process. Note that a Rhea reaction can be mapped to several EC numbers and an EC number in turn can be mapped to several Rhea reactions as the granularity of the classification is not the same for EC numbers and Rhea. We can search Rhea for both complete and partial EC numbers either using the simple search, the advanced search, or the retrieve ID mapping tool. If we do a simple search with the prefix EC colon star, we will retrieve all the Rhea reactions which have been mapped to an EC number. The links to EC numbers are provided by default in a dedicated column of the result table. We can filter any search result by enzyme class. If we click on one of the seven top-level classes of the enzyme classification, instead of the simple search, we could also have done an advanced search. To do this, we click on advanced search, then on all, we select enzyme classification and then directly type the number. As an example, we can type a partial EC number. We will retrieve all the Rhea reactions catalyzed by enzymes from the subclass described by the partial EC number 2.1.2. As we browse through our results, we can click on the EC number 2.1.2.5. A black box appears with three different links to explore related information in Uniprot or the enzyme database on XBusy. If we click on the link search chemical reactions in Rhea for this EC number, we see that it is linked to two Rhea reactions. The example nicely illustrates that an EC number can be linked to one or several Rhea reactions. We click on one of these reaction identifiers to go to the corresponding reaction page. There is a link to enzyme classification here at the top and also under cross-references. We can search Rhea with several EC numbers at a time. To do so, we can use the simple search box and combine our EC numbers with the OR operator. However, if we have more than just a few EC numbers, for which we'd like to obtain the corresponding Rhea reactions, it will be more convenient to use the ID mapping service. We click on the retrieve ID mapping link, select the identifier type, enzyme classification, copy, paste our list into the box and click on submit. All the numbers shown in this tutorial will of course change regularly depending on the evolution of biological knowledge and on progressing bio-curation efforts. Never hesitate to contact us if you have any questions or remarks and to look at our help pages and other videos. Thank you very much for your attention.