Isopentenyl pyrophosphate (IPP, isopentenyl diphosphate, or IDP) is an intermediate in the classical, HMG-CoA reductase pathway (commonly called the mevalonate pathway), and is used by organisms in the biosynthesis of terpenes and terpenoids. IPP is formed from acetyl-CoA via the mevalonate pathway (the "upstream" part), and then is isomerized to dimethylallyl pyrophosphate by the enzyme isopentenyl pyrophosphate isomerase.