Resorufin benzyl ether is a fluorometric probe that acts as a substrate for cytochrome P450 (CYP)3A4. It is typically used near its apparent K value of 30 µM to screen the inhibition/activation potential of test compounds, to predict potential drug-drug interactions, or to monitor other CYP450 activities. Upon enzyme cleavage, resorufin is released and displays excitation/emission maxima of 570/580 nm, respectively.