Number 173 (Story #3), April 12, 1994 by Phillip F. Schewe and Ben Stein MOLECULAR LEVEL INTERNAL COMBUSTION ENGINES , convert chemical energy into mechanical motion by burning single molecules. No such motor has yet been made artificially, but examples abound in biology, where the "fuel" is adenosine triphosphate (ATP), the basic energy-carrying molecule in cells. Two examples of "protein motors" are myosin, which slides past actin filaments to produce muscle contraction, and kinesin, which transports material in cells. The size of the kinesin motor is 12 nm, 50 times smaller than the smallest transistor now made. Marcelo Magnasco (212-327-8542) of Rockefeller University and NEC Research Institute has developed a general framework for such motors which describes the relationship between their state of motion and the rate at which they consume chemical fuel. Magnasco's description paves the way for a fundamental, physics-based understanding of motor proteins and provides insights into designing artificial ones. (Upcoming article: 18 April, Physical Review Letters.)