Expansion of MUC1 tetramer–positive cells after vaccination. CD8⁺ T cells binding the MUC1 tetramer were quantified at serial time points (1, before first vaccination; 2, before second vaccine; and 3, before third vaccination) in patients who were HLA-A2.1. Binding to a control tetramer was quantified in parallel, and the control value was subtracted from that obtained for the MUC1 tetramer. Mean values of 5 patients are presented with associated standard error. An incremental, although not statistically significant, increase in MUC1 tetramer–positive cells was observed after vaccination.