Orphan enzyme activities correspond to the enzyme activities (EC numbers) defined by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB), and which are not associated with any amino acid sequences in the major public databases (i.e. UniProt, Enzyme, PDB, IntEnz, KEGG, BRENDA). These are cases of "missing genes" for known enzymes.
ORENZA is a relational database of ORphan ENZyme Activities and this web site allows you to easily browse and search ORENZA.