Two-dimensional flow cytometric analysis of a cell culture induced to undergo apoptosis, using annexin V affinity labeling of PS and PI uptake for DNA staining, allowing the distinction of viable, apoptotic, and dead cells. Viable cells show neither PI nor annexin V staining, whereas apoptotic cells are exclusively stained by annexin V. Necrotic (dead) cells and cells damaged during the isolation procedure become positive for both annexin V and PI. Reproduced from van Engeland et al⁴³  with permission.