roGFP2-Orp1 is a genetically encoded fluorescent sensor that allows real-time measurements of intracellular hydrogen peroxide under physiologically relevant conditions. It is a fusion protein composed of roGFP2 (a redox-sensitive form of GFP which is insensitive to pH changes in the physiological range) fused to the thioredoxin-dependent peroxidase encoded by the Saccharomyces cerevisiae HYR1 gene (SGD:S000001476, also known as 'Orp1, oxidant receptor peroxidase 1') (PMID:19755417). For detailed kinetic and spectral properties, see PMID:19755417.