Signal from the selenenylsulfide probes was strong in different cell lines (Fig. ), dose-dependent (Fig. ), stable over long timecourses, and not due to direct carbamate cleavage (C1, Fig. ). The potentially TrxR-selective RX1 was predictably slower to generate signal than nonselective disulfide SS00 or GSH-sensitive G1. Fluorescence imaging showed near-ubiquitous turn-on of RX1 with PQ-OH fluorophore being cellularly retained for hours (Fig. ). We then performed several orthogonal tests of whether cellular activation of the RX1 probe is indeed TrxR-selective.