The Aft (anthocyanin fruit) gene, from S. chilense, is a regulatory gene (An2 transcription factor) that is induced by UV light and that triggers up-regulation of anthocyanin biosynthesis in tomato fruit, and to a lesser extent in vegetative tissue. In Aft plants anthocyanin accumulates in and just below the fruit epidermis, where light penetrates the first few layer of cells. It is believed that such accumulation is a stress and protection response to UV light (think melanin accumulation in human skin cells in response to UV light – tanning). Typically those parts of the fruit exposed to light will accumulate anthocyanin and turn black/purple, while those parts of the fruit not exposed to light will have the normal background fruit color. Anthocyanin accumulation begins very early during the green fruit stage and masks normal fruit color in mature fruit. Anthocyanin appears to accumulate preferentially in the green stripes of gs and Fs fruit.