NucPE1 (Nuclear Peroxy Emerald 1) is a nuclear-localized fluorescent hydrogen peroxide that is specifically localized to cellular nuclei without appended targeting moieties. In Vitro NucPE1 features two major visible region absorptions (λ abs =468 nm, ε=27,300 M -1 cm -1 ; λ abs =490 nm, ε=26,000 M -1 cm -1 ) and a weak emission (λ em =530 nm, Φ=0.117). Reaction of NucPE1 with H 2 O 2 triggers a fluorescence increase upon its conversion to fluorophore NucPE1, which possesses one major absorption band at 505 nm (ε=19,100 M -1 cm -1 ) with enhanced emission (λ em =530 nm, Φ=0.626). NucPE1 selectively accumulates in the nuclei of a variety of mammalian cell lines as well as in whole model organisms like C. clegans , where it can respond to subcellular changes in H 2 O 2 fluxes. MCE has not independently confirmed the accuracy of these methods. They are for reference only. In Vivo NucPE1 maintains the ability to selectively target nuclei in vivo . NucPE1 imaging reveals a reduction in nuclear H 2 O 2 levels in worms overexpressing sir-2.1 compared to wildtype congeners, supporting a link between this longevity-promoting sirtuin protein and enhanced regulation of nuclear ROS pools . MCE has not independently confirmed the accuracy of these methods. They are for reference only. Form:Solid