Plasminogen retards DNA fragmentation. Plasminogen was preincubated with U937 cells for 30 minutes. Without washing the cells, TNFα and CHX were added, and the cells were incubated for an additional 24 hours. DNA was extracted as described in “Materials and methods” and electrophoresed on a 1.5% low-melt agarose gel. FACS analysis was performed simultaneously to verify the percentage of apoptosis within the total cell population.