Faithful activation of an extra-bright red fluorescent protein in "knock-in" Cre-reporter mice ideally suited for lineage tracing studies.

The considerable potential of Cre recombinase as a tool for in vivo fate-mapping studies depends on the availability of reliable reporter mice. By targeting a tandem-dimer red fluorescent protein (tdRFP) with advanced spectral and biological properties into the ubiquitously expressed ROSA26 locus of C57BL/6-ES cells, we have generated a novel inbred Cre-reporter mouse with several unique characteristics. We directly demonstrate the usefulness of our reporter strain in inter-crosses with a "universal Cre-deleter" strain and with mice expressing Cre recombinase in a T lineage-specific manner. Cytofluorometric and histological analyses illustrate: (i) non-toxicity and extraordinary brightness of the fluorescent reporter, allowing quantitative detection and purification of labeled cells with highest accuracy, (ii) reliable Cre-mediated activation of tdRFP from an antisense orientation relative to ROSA26 transcription, effectively excluding "leaky" reporter expression, (iii) absence of gene expression variegation effects, (iv) quantitative detection of tdRFP-expressing cells even in paraformaldehyde-fixed tissue sections, and (v) full compatibility with GFP/YFP-based fluorescent markers in multicolor experiments. Taken together, the data show that our C57BL/6-inbred reporter mice are ideally suited for sophisticated lineage-tracing experiments requiring sensitive and quantitative detection/purification of live Cre-expressing cells and their progeny.