Taken together, these four aspects informed our hypotheses that probes of type A (which includes the RX1 probe) will be: (1-2) rapidly attacked by TrxR, (2) much more efficiently reduced through to C by TrxR than by thiols, (3) resistant to on-reductant signal generation by (mono)thiols, and (4) rapidly cyclise after reduction to C. In contrast, though probes of type G will share features (1-2), they (3) should be labile to thiols: thus acting as a stringent control for our mechanism-based design. Probe design. We designed selenenylsulfide-based probes that use carbamate cyclisation to release phenolic cargos (Fig. ). This is a modular system that can be applied to any phenol-type cargos, including drugs. The phenolic carbamate system offers a range of desirable features, reviewed elsewhere; important aspects are: (i) phenolates are good leaving-groups, giving fast cargo release, that is also orders of magnitude faster than with aniline carbamates that are more often 36 used. (ii) Though primary amine phenolic carbamates are so unstable due to E1cB elimination that they are usually discarded (halflife ~seconds ), secondary amine carbamates avoid E1cB and such probes are very robust to spontaneous hydrolysis (only ca. 1% in 3 weeks). (iii) Masking the hydroxyl of many phenols blocks their activities; so this system allows true off-to-on performance (zero activity before release) with a range of cargos. For example, phenol O-unmasking is required for activation of masked fluoresceins, indigo-type chromophores, luciferins, and bioactivity in many series of drugs (irinotecan 48 , duocarmycin ). Thus, it should be possible to extend this work's modular design to probes with diverse imaging modalities, and to many prodrug types.