We measured the inversion rate in the Cre-DNA–injected embryos used for the experiments in Figure 3, as mentioned in the subsection “Cre DNA injection” of the Materials and methods in the original manuscript. We used a PCR-based approach for this measurement, as it was difficult to prepare an adequate quantity of genomic DNA from embryos for Southern blot analysis. We have included a new subsection titled “Inversion rate measurement” in the Materials and methods in the revised manuscript and described how the inversion rate of the gene trap cassette was measured in embryos and epicardial cells. We presented the data in Figure 4—figure supplement 2.