Time course of apoptosis and PS externalization in U937 cells. U937 cells were treated with 1 (circles), 3 (squares), and 10 (triangles) μg/mL etoposide for the indicated times. After the treatments, the development of apoptosis (open symbols) and PS externalization (solid symbols) were analyzed using a flow cytometer, as described in the Materials and Methods. The results are representative of three independent experiments.