Photocontrol of CreERT2 recombinase activity in Tg reporter line. Transgenic (ef1α:loxP-GFP-loxP-dsRed2) embryos were injected with 3 pg of CreERT2 mRNA at the one-cell stage and further incubated with 3 μM of caged inducer (cInd). Photoactivation of cInd at the 3/6-somite stage resulted in dsRed expression observed here at 2 dpf. In epifluorescence microscopy, the non-UV illuminated embryos (A) do not express dsRed, whereas they do express dsRed in every cell upon global UV illumination (B), in few cells upon twophoton excitation in a single cell in the forming retina (D), or in a just formed somite (E). Scale bars: 100 μm. Kinetics of recombination was determined by quantitative PCR (C). Genomic DNA was prepared from 30 tg (ef1α:loxP-GFP-loxPdsRed2) embryos; Green: NI, noninjected embryos; blue: embryos injected at one-cell stage with 30 pg cre-ERT2 mRNA, incubated with 3 μM of caged inducer (cInd), illuminated at 50% epiboly, and then fixed for DNA purification; red: embryos injected at one-cell stage with 30 pg cre mRNA and allowed to develop until 24 hpf. The average values (±standard error of the mean) from at least three independent experiments are presented; comparisons with t=0 were performed by a Student’s t-test. Values of p < 0.05 were considered to indicate statistical significance (***p < 0.001; **p < 0.01).