Schematic of fluorescent MHC class I–peptide tetramer used in the study by Trudeau and colleagues (1) to enumerate autoreactive CD8⁺ T cells in the peripheral blood of NOD mice. The percentages of CD8⁺ cells specific for H-K^d/NRP-V7 peptide in multiple peripheral blood samples taken from each mouse were determined by flow cytometry. The patterns for mice that went on to develop diabetes (left) and mice that failed to develop diabetes by 32 weeks of age (right) are shown in the lower histograms.