Figure 3C shows the HyPer response in cytosol before and after addition of 20 μM H2O2 and subsequent reduction with 1 mM DTT. No significant difference in fluorescence was observed between selenium-deficient and -supplemented cells. Using HyPer targeted to different compartments we also tested the redox state of this sensor in other compartments in cells subjected to selenium deficiency (Figure S2). Our data suggest that the decrease in GPx1 levels in HEK 293 cells by selenium deficiency did not alter H2O2 levels in the cellular compartments. Here, GPX1 is linked to selenium deficiency.