To increase the mosaicism of Cre expression, pUbb-iCRE-GFP DNA was co-injected with I-SceI into one-cell–stage embryos as previously described (Thermes et al., 2002). The injected embryos were examined at 3 dpf using an MVX10 microscope (Olympus, Tokyo, Japan). Embryos that expressed EGFP on approximately >80% of the total body surface area were selected for qRT-PCR and phenotypic analysis (Figure 3A–C). The embryos expressing EGFP at these levels exhibited the Zwitch inversion at more than 90% efficiency (Figure 4—figure supplement 2C).