Transgenic mouse model exhibiting weak red fluorescence before and strong green fluorescenceafter Cre/loxP-mediated recombination.

The Cre/loxP system is an indispensable tool for temporal and spatial control of gene function in mice. Many mice that express Cre and carry loxP sites in their genomes have been bred for functional analysis of various genes in vivo. Also, several reporter mice have been generated for monitoring of recombination by the Cre/loxP system. We have developed a Cre reporter gene with DsRed1 and Venus that exhibits a strong red fluorescence before and a strong green fluorescence after Cre/loxP-mediated recombination in experiments using NIH3T3 cells. However, a transgenic mouse introduced with the same reporter gene exhibits a weak red fluorescence before and a strong green fluorescence after Cre/loxP-mediated recombination. This property manifested ubiquitously in this mouse model and was maintained stably in mouse-derived fibroblasts. Use of the mouse model exhibiting the stronger red fluorescence might result in confusion of the Cre-dependent signal with false signals, because the Venus signal includes some fluorescence in the red region of the spectrum and the DsRed1 signal includes some fluorescence in the green region. However, we fortuitously obtained reporter mice that exhibit a weaker red fluorescence before Cre/loxP-mediated recombination. The use of this mouse model would decrease concern regarding errors in the identification of signals and should increase certainty in the detection of Cre activity in vivo.