 This limits the usefulness of "S. cerevisiae" in the production of fuels and chemicals from plant cell walls, which contain a large amount of xylose.In response, "S. cerevisiae" has been engineered to ferment xylose through the addition of the "S. stiptis" genes, XYL1 and XYL2, coding for xylose reductase and xylitol dehydrogenase, respectively.The concerted action of these enzymes converts xylose to xylulose, which is naturally fermented by "S. cerevisae".