The level of intracellular ROS was measured by the alteration of fluorescence resulting from oxidation of 2', 7'-dichlorofluorescein diacetate (DCFH-DA, Molecular Probes, Eugene, OR). DCFH-DA was dissolved in DMSO to a final concentration of 20 mM before use. For the measurement of ROS, cells were treated with Q7R and other reagents for a time period indicated in the figure legends. After washing twice with cold PBS, they were incubated with 20 μM DCFH-DA at 37°C for 15 min. DCFH-DA is a stable compound that easily diffuses in to cells and is hydrolyzed by intracellular esterase to yield a reduced, non-fluorescent compound, DCFH, which is trapped within cells. The ROS produced by cells oxidized the DCFH to highly fluorescent 2', and 7'-dichlorodihydrofluorescein (DCF). The intensity of fluorescence was recorded using a flow cytometry (Becton Dickenson), with an excitation filter of 530 nm and an emission filter 575 nm. The ROS level was calculated as a ratio of: ROS = mean intensity of exposed cells: mean intensity of unexposed cells.