Detection of superoxide in live cells using MitoSOX™ Red superoxide indicator (M36008). Live 3T3 cells were treated with FeTCPP, a superoxide scavenger, (bottom) or left untreated (top). Cells were then labeled with MitoSOX™ Red reagent, which fluoresces when oxidized by superoxide, and nuclei were stained with blue-fluorescent Hoechst 33342. The mitochondria of untreated cells exhibited red fluorescence, indicating the presence of superoxide, whereas the mitochondria of treated cells showed minimal fluorescence.