Diterpenes are a class of chemical compounds composed of two terpene units with the molecular formula C₂₀H₃₂; they may also be thought of as consisting of four isoprene units. They are biosynthesized by plants, animals and fungi via the HMG-CoA reductase pathway, with geranylgeranyl pyrophosphate being a primary intermediate. Diterpenes form the basis for biologically important compounds such as retinol, retinal, and phytol. They are known to be antimicrobial and antiinflammatory.