Because the specific activity of cellulase enzymes is at least two-fold lower than that of other hydrolytic enzymes such as starch-hydrolyzing enzymes (Zhang and Lynd 2004; Wilson 2008), even native cellulolytic organisms must produce a high titer of cellulase to efficiently hydrolyze cellulose. The need for synthesis of a large quantity of cellulases is a "metabolic burden," even to native cellulolytic organisms (Zhang and Lynd 2005). Thus, heterologous expression of cellulolytic enzymes in industrial ethanol-producing hosts such as S. cerevisiae,

E. coli and Z. mobilis is especially challenging. Despite these obstacles, several recombinant strains have been engineered for efficient cellulosic ethanol production.