Cellular quantitative high-throughput screening. 5 μL of A549 cell suspension diluted to 1250 cells/well were dispensed into 1536well black/clear flat bottom TC-treated microplates (Corning) using a Multidrop TM Combi Reagent Dispenser. Following 4 h incubation at 37 °C to allow cells to adhere, 25 nL of LOPAC1280 compounds or controls (in DMSO) were spotted into wells using an Echo 525 Acoustic liquid handler, and cells were incubated for a further 1 h. Then, 1 μL of diluted RX1 were dispensed into each well using a BioRAPTR 2 FRD Dispenser (Beckman); plates were imaged at 1 h time intervals using an Envision plate reader (λex = 340/25 nm, λem = 520/25 nm, 10 flashes, bottom read) during 4 h further incubation (plates kept at 37 °C between readings). L.Z. thanks the Fonds der chemischen Industrie for generous financial support through a Kekulé fellowship. J.G.F. thanks the Studienstiftung des deutschen Volkes for support through a PhD scholarship. L.P. thanks the GRK 2338 for support through a PhD scholarship. J.T.-S. thanks the Joachim Herz Foundation for fellowship support. We thank Marcus Conrad (Helmholtz Centre, Munich, DE) for MEF and knockout MEF cell lines; Carina Schmitt (LMU) for synthetic assistance; and Thomas Gudermann (LMU) for support through the GRK 2338. We thank the attendees of the SPP 1710 conference Thiol-Based Redox Switches in 2019, and in particular Christopher Lillig (Greifswald, DE), for their supportive, collegial, and helpfully challenging discussions. This paper is dedicated to Yasuteru Urano, whose rigorous and farreaching developments towards translational applications of chemical biology probes are inspiring to us and to the field.