10-Acetyl-3,7-dihydroxyphenoxazine (Known as Amplex Red? reagent, Trade Mark of Molecular Probes) is regarded as the best fluorogenic substrate for peroxidase, because it is highly specific and stable. The substrate itself is nearly colorless and nonfluorescent until it is oxidized by H2O2 in the presence of horseradish peroxidase (HRP) to become the highly red fluorescent resorufin. Because H2O2 is produced in many different enzymatic reactions, the Amplex Red reagent can be used to detect the activity of many different enzymes.