a large class of natural compounds formed in organisms from mevalonic acid. The latter is converted in the microsomes of the cells into “five-carbon fragments” with an isoprene skeleton. The biosynthesis of isoprenoids is a process of consecutive linking (condensation) of such five-carbon units into chains of various lengths. The doubling, cyclization, oxidation, reduction, and regrouping of such chains accounts for the extraordinary structural diversity of isoprenoids. The isoprenoids include terpenes and their derivatives, sterols, steroids, carote-noids, xanthophylls, and the polyisoprenoids—natural rubber and gutta-percha. A number of isoprenoids have great biological significance—many hormones of animals, plants, and lower organisms; certain vitamins; antibiotics; and attractants.