PierO (Enzyme Reaction Ontology for Partial Information) intends to describe enzyme reactions even if the only partial information is available, aiming at the use for metabolomics studies. It contains hierarchical classification and ontology of functional groups and chemical substructures, which is originally obtained from BiSSCat database (http://bisscat.org/) and has been modified. ids:ec-code ids:go piero:chemical_structure_information piero:chemical_substructure piero:ec piero:enzyme_function piero:enzyme_information piero:gene_product piero:go piero:iubmb piero:kcfs piero:molecular_function piero:molecule piero:object piero:process piero:rdm piero:reaction piero:reaction_information piero:reaction_ontology_information piero:substructure piero:substructure_conversion piero:transformation kcfs:atom kcfs:bond kcfs:inorganic kcfs:ring kcfs:skeleton kcfs:triplet kcfs:vicinity kegg:compound kegg:enzyme kegg:ko kegg:rclass kegg:reaction kegg:rpair owl:Thing PierO (Enzyme Reaction Ontology for Partial Information) intends to describe enzyme reactions even if the only partial information is available, aiming at the use for metabolomics studies. It contains hierarchical classification and ontology of functional groups and chemical substructures, which is originally obtained from BiSSCat database (http://bisscat.org/) and has been modified.