From Jackson Labs: These R26-stop-EYFP mutant mice have a loxP-flanked STOP sequence followed by the Enhanced Yellow Fluorescent Protein gene (EYFP) inserted into the Gt(ROSA)26Sor locus. When bred to mice expressing Cre recombinase, the STOP sequence is deleted and EYFP expression is observed in the cre-expressing tissue(s) of the double mutant offspring. These mutant mice may be useful in monitoring the Cre expression in living tissues, and tracing the lineage of such cells in embryos, young, and adult mice at desired time points.