Aminophenylfluorescein (APF, A36003) and Hydroxyphenylfluorescein (HPF) They are two reactive oxygen species (ROS) indicators developed by T. Nagano, which have higher specificity and stability than dichlorodihydrofluorescein diacetate (H₂DCFDA, D399) . APF and HPF have limited non-selective reactivity and a relatively high tolerance to light-induced oxidation. These two new fluorescein derivatives do not fluoresce until they react with hydroxyl radicals or peroxynitrite anions. APF also reacts with hypochlorite anions. Both ROS indicators can be used together to selectively detect hypochlorite anions. Upon oxidation, APF and HPF produce bright green fluorescence (maximum excitation/emission wavelength ∼490/515 nm) that is compatible with any instrument that detects fluorescein.