Thalidomide induces intracellular ROS in erythroid progenitor cells. CD34⁺ cells were treated with the indicated concentration of thalidomide (A) or with 100 μM thalidomide in the presence or absence of catalase (4000 U/mL), DMTU (10 mM), or SB203580 (5 μM) (B) from day 3 to day 5 for 48 hours, followed by treatment with the DCFDA probe for 10 minutes. The intracellular reactive oxygen species (ROS) production was measured by flow cytometry. ROS levels induced by thalidomide were normalized to those of untreated cells as determined by monitoring the increased fluorescence in the cells. Results are shown as means (± SD) from 3 independent experiments. *P < .05 versus untreated cells.