Grape skin, the outer layer of the grape (Vitis vinifera), is either green, red, or purplish-black in color. The skin, stem, seeds, and juice of the grape are used in making wine. Although the skin, stem, and seeds are often used in making the nutritional supplement, grape skin extract, the extract sometimes contains grape skin only. Generally, the skin of red grapes is used in making nutritional supplements. Red grape skins contain an array of bioflavonoids (quercetin, catechins, flavonols, and anthocyanidins) and nonbioflavonoid polyphenols (acid derivatives). One important nonbioflavonoid in grape skin is called resveratrol. Resveratrol is a plant-specific enzyme that exists in 72 plant species, such as grapes, peanuts, and pine trees. Grapes are the most abundant source of this health-promoting enzyme. Resveratrol's presence in the plant is induced by stress, injury, infection or ultraviolet irradiation. It is thought that the injury to the grape skin, produced during the wine-making process, significantly increases resveratrol levels. The relatively high quantities of the enzyme in the grape skins are thought to help the plant resist fungal infections, diseases, adverse weather, and insect or animal attack.