Persistence of cytomegalovirus (CMV) and Epstein-Barr virus (EBV)-specific CD8 T cell clones, using allophycocyanin (APC)-labeled HLA-viral peptide tetramer analysis. Method: Both patients were positive for HLA-A*0201. Thus, we used NLVPMVATV (CMV peptide) conjugated to HLA-A*0201-APC tetramer and GLCTLVAML (EBV peptide) conjugated to HLA*0201-APC tetramer (both purchased from Beckman Coulter) to stain CMV and EBV-specific clones in blood mononuclear cell specimens by flow cytometry. The cells were also stained by CD3-phycoerythrin antibody and CD8-FITC antibody. The density dotplots, showing tetramer-APC fluorescence on the x-axis and CD8-FITC fluorescence on the y-axis, were gated on CD3+CD8+ lymphocytes. The CMV tetramer staining of the cells from the MS patient, who was CMV-seronegative, served as a negative control for setting the border between tetramer+ and tetramer- cells. The percentages of the tetramer+ cells (of total CD8 T cells) are shown, indicating that even though the percentages varied, the same EBV-specific CD8 clones in both patients and the same CMV-specific CD8 clone in the CMV-seropositive patient were present both pretransplant and posttransplant.