Isoprenoids are synthesized by consecutive condensations of their five-carbon precursor, isopentenyl diphosphate, to its isomer, dimethylallyl diphosphate. Two pathways for these precursors are known. One is the mevalonate pathway, which operates in eucaryotes, archaebacteria, and cytosols of higher plants. The other is a recently discovered pathway, the nonmevalonate pathway, which is used by many eubacteria, green algae, and chloroplasts of higher plants. To date, five reaction steps in this new pathway and their corresponding enzymes have been identified. EC numbers of these enzymes have been assigned by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB) and are available at http://www.chem.qmw.ac.uk/iubmb/enzyme/reaction/terp/nonMVA.html.