High expression of XYL2 coding for xylitol dehydrogenase is necessary for efficient xylose fermentation by engineered Saccharomyces cerevisiae

► Low expression levels of the XYL2 might be a major reason of reported variations in xylose fermentation. ► As the expression levels of XYL2 increased, xylitol yields decreased significantly. ► Strong expression of XYL2 is a necessary condition for efficient xylose fermentation.