Cytotoxicity of etoposide on primary human CD34⁺ cells. (A) Pulse field electrophoresis of genomic DNA to detect DNA fragmentation following 20 μM etoposide exposure and 0 hour to 24 hours recovery times. Fragment populations of more than 400 kb and 50 kb are indicated on the left. (B) Annexin V detection of apoptotic cells. Black line histogram indicates unstained control; grey histogram, annexin V–positive population. Left panel: untreated CD34⁺ cells. Right panel: CD34⁺ cells following 20 μM etoposide exposure for 24 hours. (C) Recovery and proliferation of CD34⁺ cells in culture after exposure to etoposide. There were 3 independent experiments performed, and standard deviations indicated by error bars. The difference in expansion between the untreated and treated samples is statistically significant on days 1, 2, and 7. Grey bars indicate untreated CD34⁺ cells; black bars, CD34⁺ cells following 20 μM etoposide exposure.