Hyperdiploid lymphoblasts undergo apoptosis despite the presence of stromal layers. Flow cytometric dot plots represent assessment of apoptosis before (top panels) and after 2 days of culture (bottom panels) in one case of hyperdiploid ALL (55 chromosomes). Left panels illustrate FSC (a measurement of cell size) versus SSC (a measurement of cell granularity). Center panels illustrate binding of Annexin V FITC (an indicator of apoptosis) and staining with propidium iodide (PI; a sign of membrane permeability). Right panels illustrate the cells’ DNA content, measured by staining with PI after cell membrane permeabilization, and labeling with dUTP-BrdU, which is incorporated into cells with fragmented DNA. In all panels, events outside the gates correspond to cells at various stages of apopotic death.