Frequency of EBNA1-specific proliferating CD4⁺ T-cell precursors. Each data point represents the frequency of CD4⁺ T-cell precursor cells from the original culture from each volunteer relative to one of 3 stimuli. The stimuli are indicated along the x-axis: medium (nothing added), CMV (HCMV-derived CD8⁺ T-cell epitopes; Table S1), and EBNA1 (the master mix of all 51 overlapping peptides of EBNA1; Table S1). The frequency of precursors from each individual is indicated by a diamond. The average frequency of precursors from each of the 3 stimuli is indicated by the black bars.