Plant biomass is usually pretreated and broken down (saccharified) into a mixture of simple sugars (also called "monosaccharides") before fermentation into ethanol. The yeast, Saccharomyces cerevisiae is the more popular microorganism for fermentation of the monosaccharide mixture. The problem with ethanol fermentation by S. cerevisiae is that the yeast only utilizes the glucose (the six-carbon sugar) in the monosaccharide mixture. Other non-glucose monosaccharides (such as the five-carbon sugar, xylose), can potentially be converted to ethanol, but these are left unconverted by S. cerevisiae. Strategies for the utilization of unconverted xylose to ethanol by novel or improved strains of S. cerevisiae could help increase cellulose ethanol yields. Until recently, however, molecular biology attempts to insert xylose-conversion enzymes into S. cerevisiae have reportedly been not very successful. German scientists (from the Institute of Molecular Biosciences, Goethe-University Frankfurt) recently reported the successful cloning and expression of a "highly active new kind of xylose isomerase from the anaerobic bacterium Clostridium phytofermentans in S. cerevisiae". Their findings are reported in the journal, Applied and Environmental Microbiology (URL above). By having a strain of S. cerevisiae which can convert both glucose and xylose into ethanol, the product yield could increase and the production cost of cellulose ethanol could decrease..