Campesterol (3) and dihydrobrassicasterol (4), typical C28-sterols in higher plants, are biosynthesized from a steroidal 24-ene precursor (desmosterol 1) via 24-methylenecholesterol (2) and 24-methyldesmosterol (5). A typical plant C29-sterol, sitosterol (6), is produced from 24-methylenecholesterol