a, To induce Polylox recombination in different tissues in vivo, we crossed the Rosa26Polylox allele into mice bearing the Rosa26CreERT2 allele, which encodes ubiquitously expressed, tamoxifen-regulated Cre, yielding Rosa26Polylox/CreERT2 mice. b, Adult Rosa26Polylox/CreERT2 mice were injected with tamoxifen or with oil only (vehicle control) according to the depicted schedule, and were analysed on day 5. c, Genomic DNA was prepared from indicated organs that represent developmental derivatives of all three germ layers: brain (ectoderm), muscle, spleen, and thymus (mesoderm), and liver and lung (endoderm). The Polylox cassette was amplified by PCR, and recombination in each tissue and for all time points was visualized by separating DNA fragments by gel electrophoresis (Supplementary Fig. 1f). The first lane is the PCR water control, the second lane is from Rosa26+/+ DNA template, and the third lane is from Rosa26Polylox(no Cre) template; all other lanes show data from Rosa26Polylox/CreERT2 mice for the indicated organs and conditions. The DNA sample and PCR result from the muscle oil control were not available.