Tocopherols and tocotrienols constitute a series of related benzopyranols (or methyl tocols) that are synthesised in plants and other photosynthetic organisms, where they have many important functions. First described in 1922 as a dietary factor essential to prevent fetal reabsorption in rats, it was soon understood that they contained a vitamin (vitamin E) that is essential for innumerable aspects of animal development. Tocopherols are now known to be powerful lipid-soluble antioxidants, but only one isomer, i.e. α-tocopherol, is recognized as having vitamin E activity. In addition, this has regulatory roles in signal transduction and gene expression in animal tissues. Vegetable oils are a major dietary source.