Led by Ken Coppieters, now at Ghent University in Belgium, La Jolla researchers attached green and blue fluorescent proteins to cytotoxic T cells, a type of white blood cell, and transplanted them into a mouse. The cells migrated randomly throughout the pancreas, making their way to beta cells and eventually killing them. Though they moved in random patterns, the cells were observed to accumulate within and around the β cell population, the authors say. They moved at an average 10 microns per minute and up to 25 microns per minute — not the world's fastest cells by a long stretch. This comparatively slow process could be one reason why diabetes takes a while to present itself clinically — by the time a patient is diagnosed, up to 90 percent of his or her beta cells have already been destroyed.