Stable integration of the lentivirus maintained throughout human B-cell development in vitro. Genomic DNA from cells at week 2 and week 4 of stage 2 (representing progenitor and pre-B stages, respectively) and at the end of stage 3 (representing plasma cell stage) was extracted and virus integration was determined as described in “Detection of virus integration.” Averaged provirus copy numbers per cell are shown for no virus or different lentiviral constructs originally transduced into CD34⁺ cells. The results are shown as mean plus SE from combined data obtained from experiments for the same transgene (vector only, GFP, or b12; regardless of U, MH, or EEK promoters). There were 5, 3, 6, and 6 biologic replicates for no virus, vector, GFP, and b12, respectively. The provirus copy numbers per cell at plasma cell stage, on average, were not statistically different from those of their progenitors (P = .26). Student 2-tailed t test was performed using Excel statistical software module (Microsoft, Redmond, WA), and the difference was considered statistically significant when the P value was less than .05.