Butanol (a 4-carbon alcohol) is considered an “advanced biofuel”, and is reported to have better biofuel features compared to its 2-carbon relative, ethanol. Although butanol has better energy density, better transportability in existing pipeline structures, and can be used in unmodified vehicles at higher blends (compared to ethanol), there are still some bottlenecks for industrial scale production. Butanol is mainly produced by bacteria of the genus Clostridium, under anaerobic conditions. Bacterial production of butanol by Clostridia is not considered a good method because of slow bacterial growth, bacterial toxicity toward the butanol product at 1% to 2% concentrations, and production of significant quantities of unwanted by-products such as acetone. On the other hand, Saccharomyces cerevisiae, the yeast commonly used for ethanol production, grows faster, has a higher alcohol tolerance and produces relatively less unwanted by-products. Scientists from the University of California Berkeley attempted to metabolically engineer Saccharomyces cerevisiae with an n-butanol biosynthetic pathway, in which isozymes from a number of different organisms were substituted with Clostridial enzymes. They evaluated the effect of the substitution on n-butanol production. Results showed that expression of the genes encoding the butyryl-CoA dehydrogenase from C. beijerinckii did not improve butanol production significantly. The problematic steps in the n-butanol biosynthetic pathway and those which could be taken for future improvement were identified..