Long-term functionality. Change in the frequency of gene-marked cells in 46 peripheral blood samples from 18 patients before and after ex vivo stimulation with EBV-LCLs. The median marking efficiency before prestimulation was 0.0008% (SEM = 0.0007%) compared with 1.49% (SEM = 0.40%) after stimulation (P < .001 by the Wilcoxon signed-rank test). In 20 samples that were initially negative for gene-marked cells, ex vivo stimulation with EBV-LCLs expanded the marked population to a level above the 1/10 000 threshold for quantitative RT-PCR detection.