The yeast, Saccharomyces cerevisiae, has been a traditional organism for ethanol production (from saccharine substrates, like glucose or sucrose), way before the "biofuel ethanol age". Until recently, ethanol has been produced by fermentation, mainly for beverage purposes. With the coming of the "biofuel ethanol age" and the trend towards the use of lignocellulosic biomass as "second-generation ethanol feedstock", scientists have begun to provide Saccharomyces cerevisiae with the metabolic flexibility to utilize (and ferment) two major sugars that are present in pretreated lignocellulosic biomass: (1) glucose and (2) xylose. Native Saccharomyces cerevisiae is mainly a "glucose utilizer", and it does not have a metabolic capability to utilize xylose, an abundant pentose (i.e. 5-carbon) sugar in pretreated lignocellulosic biomass. Molecular biology techniques have been able to develop recombinant S. cerevisiae strains which can utilize and ferment xylose to ethanol. However, many of these strains have low fermentation yields. One problem reportedly lies in the transport of xylose to the cell. Lund University (Sweden) scientists report that in xylose-utilizing recombinant S. cerevisiae strains, xylose is "transported by non-specific hexose transporters with poor affinity for xylose". The inefficient transport of xylose in the cell inevitably results in low utilization and low ethanol yields. There have been recent reports of improved transport and utilization of xylose by "independent expression of heterologous transport", (Gxf1, Sut1, and At5g5920)". However, the studies were reported in different strains. Lund University scientist, David Runquist and colleagues investigated the three transporters under identical growth conditions. A direct relationship between the transport kinetics of the individual transporters and xylose utilization was observed, under "transport limited conditions". The results also "point to the importance of designing an appropriate co-fermentation strategy for industrial ethanol fermentation from lignocellulosic biomass". The full study is published in the online access journal, Biotechnology for Biofuels (URL above)..