Blood mix-ups, though rare, are still one of the most feared mistakes in transfusion medicine. "It's the biggest threat today," says Dr. Kathleen Sazama, a transfusion expert at the University of Texas M.D. Anderson Cancer Center in Houston. Even an ounce of mismatched cells can trigger a potentially lethal immune response, causing blood clots and internal bleeding. But now scientists at a Massachusetts biotech firm may be on the brink of eliminating most transfusion errors and ensuring a steady supply of blood to the nation's hospitals. Their solution is a device that converts all blood into type O, the most coveted of the four major blood types because it can be safely transfused into nearly any patient. "Press 'start,' and the machine does everything else," says Douglas Clibourn, the CEO of ZymeQuest.