Blood banks carefully screen patients and donors by their blood type: A, B, AB, O. They also look for rarer blood types that might lead to transfusion problems. But some blood types remain a mystery. Like Vel. Doctors dread the surfacing of a "Vel-negative." For that patient, transfusion can lead to exploding blood cells, liver failure, and death. Now, using a high-resolution mass spectrometer, UVM biologist Bryan Ballif and colleagues have explained the inner workings of Vel, solving a decades-old blood riddle.