The main raw materials for ethanol fermentation in lignocellulosic biomass are the sugars that are liberated after pretreatment and saccharification of cellulose/hemicellulose fibers. These sugars are commonly fermented to ethanol, using yeasts (Saccharomyces cerevisiae), which have been historically used for ethanol fermentation from saccharine substrates. However, only the hexoses (or six-carbon sugars), like glucose, are utilized by traditional yeast strains. The pentoses (five-carbon sugars), such as xylose and arabinose, are left unutilized, because yeasts have no natural capability to metabolize pentoses. Effective fermentation performance must involve the conversion of both pentoses and hexoses into ethanol. Recent advances in metabolic engineering have been successful in developing yeast strains with pentose-metabolizing capabilities. However, "major difficulties still remain for engineering simultaneous, exogenous sugar metabolism". Scientists from the Department of Chemical Engineering, The University of Texas at Austin (United States), review the issues related to effective pentose utilization in yeasts. Their review highlights the need to shift focus "towards nontraditional aspects of cellular engineering such as host molecular transport capability, catabolite sensing and stress response mechanisms". They termed a new approach called "panmetabolic engineering", as a new paradigm for integrating new carbon sources into host metabolic pathways. The approach is seen to simultaneously optimize "the interdependent processes of transport and metabolism using novel combinatorial techniques and global cellular engineering". The review is published in the open-access journal, Biotechnology for Biofuels (URL above).