In vitro proliferation of xenograft cells. Cell division tracking of xenograft cells was carried out exactly as described in “Materials and methods' and in the legend to Figure 6. Cells were analyzed daily after being seeded onto MS-5 cells. The proliferation index (y-axis) is plotted on a log₂ scale, and represents the fold increase in cell number by proliferation only, and does not account for cell death. ALL-7 (○) underwent the greatest amount of proliferation, followed by ALL-11 (•). ALL-17 (□) and ALL-19 (▪) exhibited only a limited propensity to proliferate during this time course. For comparison, the proliferation index of the exponentially dividing T-lineage ALL CEM cell line was 2.1, 3.3, and 3.8 at 28 hours, 45 hours, and 52 hours after sorting, respectively (data not shown).