In practical usage this is based on the specificity of the enzyme acting on its substrate, and although this can become slightly awkward, it conveys a simple and useful message. 12-LOX oxygenates arachidonic acid at carbon-12, and when necessary, the stereoconfiguration is specified (12R-LOX or 12S-LOX (Scheme FS1)). The differing chain lengths of the most common substrates of plants (linoleate, linolenate, 18-carbon) and animals (arachidonate, 20-carbon) result in a plant 13-LOX corresponding to a mammalian 15-LOX; these particular lipoxygenases “count” the substrate carbons from the tail end of the chain, and both react oxygen at the ω-6 position. Complications can arise, for example, when there is more than one 12-LOX in the same species. To get around this problem, currently the mammalian 12-lipoxygenases are named after the prototypical tissues of their occurrence (hence, the platelet, leukocyte, or epidermal type of 12-LOX (5)). These are distinct enzymes by sequence, catalytic activities, and function. Some lipoxygenases may form a mixture of products, e.g. the mammalian reticulocyte type of lipoxygenase catalyzes C-12 and C-15 oxygenation, with the relative proportions varying among species. In rabbits and humans the major product is 15-HPETE, and hence the enzyme is designated a 15-LOX. The most closely related enzyme in the rat, mouse, pig, and cow is the leukocyte type of 12-LOX, an enzyme that catalyzes mainly C-12 oxygenation with some reaction also at C-15