The red fluorescent protein variant TagRFP-T has greatly improved photostability over its parent molecule, TagRFP, but the underlying mechanism leading to this improvement is to date unknown. The 1.95 angstrom resolution crystallographic structure of TagRFP-T showed that its chromophore exists as a mixture of cis and trans coplanar isomers in roughly equal proportions. Interestingly, both isomers are able to fluoresce, a property that has never been observed in any other fluorescent protein. We propose a "circular restoration model" for TagRFP-T to explain its superior photostability: There are four co-existing chromophore states (cis/trans protonatediionized state) that can be driven by light to transform from one state into another. This model also explains how TagRPF-T essentially eliminates the temporary dark state (reversible photobleaching). (C) 2016 Elsevier Inc. All rights reserved.