(C) Scheme of the generation of transgenic Lgr5-GFPCreERT2 X Tomatof/f mice. eGFP and CreERT2 expression are controlled by the intestinal stem cell specific marker Lgr5 promoter, and the tdTomato reporter cassette is knocked into the Rosa26 locus. A floxed STOP cassette is upstream of the tdTomato gene, thus preventing tdTomato expression in the absence of tamoxifen. Upon tamoxifen exposure, CreERT2 recombinase will be translocated into the nucleus, and the STOP sequence between the loxP sites is excised and the tdTomato reporter is permanently and constitutively expressed in intestinal stem cells and their progeny. 