roGFP2-Tsa2ΔCR is an ultrasensitive genetically encoded sensor for hydrogen peroxide. 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 a typical 2-Cys peroxiredoxin encoded by the Saccharomyces cerevisiae Tsa2 gene (SGD:S000002861), in which the resolving cysteine 'C[[R]]' has been mutated. roGFP2-Tsa2ΔCR is approximately 50% oxidized under 'normal' unstressed cellular conditions and is equally responsive to increases and decreases in hydrogen peroxide, allowing fully dynamic, real-time measurement of basal hydrogen peroxide levels in living cells (adapted from PMID:27089028).