Ethanol is not the most energy-dense of fuels nor the cheapest. Consequently, Amyris Biotechnologies in Emeryville, Calif., has come up with a potentially better solution. It did so by starting with a long roster of organic compounds from which it chose potential replacements for gasoline, diesel and jet fuel that could be burned in modern engines and would be compatible with the existing petroleum infrastructure. Then the company used custom-designed microbes to produce the new fuels by fermentation from a conventional ethanol feedstock.