Most cars on the road today in the U.S. can run on blends of up to 10% ethanol (E10), and motor vehicle manufacturers already produce vehicles designed to run on much higher ethanol blends. Though E10 is mandatory only in 10 states, ethanol blends in the US are available in other states as optional or added on lower percentages as a substitute to MTBE (used to oxygenate gasoline) without any labeling, making E blends present in two-thirds of the US gas supply.