Withanolides are terpenoids, which are synthesized in plants using isoprenoids as precursors. Many sulfonic acids are prepared by hydrolysis of sulfonyl halides and related precursors. Tetraterpenes are synthesized from DOXP precursors in plants and some bacteria. Steroid biosynthesis is an anabolic pathway which produces steroids from simple precursors. Follicular DCs develop from putative mesenchymal precursors. Many laboratory methods exist for the organic synthesis of arenes from non-arene precursors. Most conductive polymers are prepared by oxidative coupling of monocyclic precursors. A related approach that also relies on molecular precursors involves chemical vapor deposition. Lipids are metabolized during the brood stage for precursors required for future biosynthesis. The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. Released sulfur dioxide is one of the primary precursors of acid rain. There are 90 known genes that encode human neuropeptide precursors. Estrogens can be produced in the brain from steroid precursors. Many kinds of plastics exist depending on their precursors and the method for their polymerization. Zymogens are the usually inactive precursors of an enzyme.