Additionally, since TrxR and GR are NADPH-driven enzymes, ratiometric monitoring of their redox poise is not informative about their biology, which depends on turnover. Measuring mRNA or protein expression levels is also insufficient to understand cellular redox systems, since reaction rates through networks are dynamically controlled on many levels e.g. by protein binding partners, post-translational modifications, subcellular localisation, and throttling flow from upstream reductants (or to downstream electron acceptors). Molecular probes that selectively report on the activity of individual redox-active proteins within the TrxR/Trx and GR/GSH/Grx networks would be ideal tools for studying redox biology: but no such probes exist.