DsRed(E5-NA) is a tetrameric, non-aggregating fluorescent timer protein. The fluorophore can mature in one of two ways: to a green fluorescent form, or to a blue fluorescent intermediate that transforms slowly into a red fluorescent form. Efficient fluorescence resonance energy transfer (FRET) occurs from the mature green to the mature red form in the tetrameric protein, quenching green fluorescence; this property, together with the different maturation rates of the various forms mean that after a pulse of expression of DsRed(E5-NA), blue fluorescence is detected initially, followed by green and then eventually red fluorescence (PMID:15217617). In addition, the choice of maturation pathway is determined by oxygen concentration; at high oxygen levels DsRed(E5-NA) matures predominantly into the red form, whereas the green form is predominant at low oxygen levels. DsRed(E5-NA) can thus be used as a sensor to record the history of oxygen exposure in vivo, at single cell resolution (FBrf0238808). DsRed(E5-NA) is an artificial derivative of the naturally occurring fluorescent protein encoded by the Discosoma drFP583 gene (GenBank:AF168419; AAF03369), derived by mutagenesis of the DsRed(E5) timer protein. It contains the mutations R2A, K5E, K9T relative to DsRed(E5), which prevent the formation of high molecular weight aggregates (PMID:11821040).