Introduction: Advances in intracellular RNA detection and imaging have been instrumental in understanding RNA localization and gene expression patterns. Methods that enable the analysis of live cells are extremely useful, especially if the cells remain accessible for further cell biological studies. Amongst the various technologies employed for live cell RNA imaging, fluorogenic oligonucleotide hybridization probes stand out, due to their ability to analyze non-modified wild-type cells. An important criterion for applications of fluorogenic hybridization probes in live cell RNA analysis is the extent of fluorescence enhancement observed upon binding of target. A less frequently considered property is the brightness in the target-bound state. This is of particular importance for imaging mRNA targets at physiologically relevant expression levels, where the challenge is to detect target-bound probe against background of cellular autofluorescence. The need for high brightness and high fluorescence turn-on upon hybridization becomes particularly evident when considering flow cytometry applications. In principle, flow cytometry would be ideally suited to analyze gene transcription of millions of cells in heterogenic populations. A fascinating vista would be to sort (and expand) cells based on detection of specific transcripts. However, while microscopic methods may be able to fade out signaling artifacts caused by mislocalization or aggregation, flow cytometry detects the entirety of the fluorescent probes. With frequently used methods such as microporation, lipofection, lipid particles or cell penetrating peptides the typical challenge is that either i) too few probes have entered the cell to generate contrast against the background of cellular autofluorescence or ii) too many probes remain unbound or mislocalize/aggregate when cell uptake is forced. Bright nanoparticle-based probes such as SmartFlares trigger endocytosis, but only a fraction of endocytosed material reaches the RNA target in the cytosol and differences in cell uptake efficiency/endosomal escape signal may become limiting. By contrast, microinjection offers full control over probe concentration inside cells, but is time consuming and only practicable for large cells. Therefore, flow cytometric detection of mRNAs in wild-type cells is typically performed after fixation, whereas live cell RNA detection with fluorogenic hybridization probes is limited to abundant targets and/or easy to transfect cells. Herein we present the detection of mRNA coding for the T cell receptor (TCR) in living, genetically nonmodified T cells by flow cytometry. Key to successful TCR mRNA detection was the introduction of light harvester dyes in RNA-based forced intercalation (FIT) probes (Fig. ). FIT probes are quencher-free oligonucleotide probes with an intercalator dye of the thiazole orange (TO) family inserted as a surrogate nucleobase that serves as a hybridization reporter. Upon target recognition, the TO-like intercalator dye is embedded within the double helical base stack Fig. ). The resulting restriction of torsional motions around the central methine bridge prolongs the lifetime of the excited state, and hence increases fluorescence. The FIT probe methodology was used previously for realtime PCR, live cell RNA imaging, RNA double strand detection, fluorogenic aptamers and was also considered for detecting RNA editing. Herein we present a new approach which involves coupling of light-collecting "auxiliary dyes" to create extremely bright FIT probes. The intercalator dyes QB, TO and a tricyclic benzothiazole containing TO derivative (TOtric) are used as hybridization reporters and acceptors of Förster resonance energy transfer (FRET), while bright Alexa Fluor (AF) dyes (568 or 488, Fig. ) with a near unity fluorescence quantum yield serve as the FRET donors. These auxiliary dyes were selected to have excitation maxima blue-shifted by only 5-20 nm relative to the hybridization reporter. This enables i) simultaneous excitation of both the auxiliary dye and the hybridization reporter with ii) efficient transfer of excitation energy to the latter along a gradient. However, emission can only occur if the torsions of the hybridization reporter dye around the methine bridge are restricted by formation of the probe-target complex. In addition, in the single stranded state, the probe can adopt conformations that allow direct contact of the two dyes. Therefore, in the absence of target, the hybridization reporter acts as quencher. In presence of target, intercalation prevents direct contact and the auxiliary dyes serve as light collectors that increase the extinction coefficient of the hybridization reporter.