We analyzed vehicle- or 4-HT–treated tcf21:CreER; shhact/ctembryos as shown in Figure 4C. We did not observe cardiac edema in the vehicle-treated embryos examined (0 abnormal in 8 analyzed) but found severe cardiac edema in six 4-HT–treated embryos (6 abnormal in 8 analyzed; p< 1.0 × 10-8, Fisher’s exact test). We have included this information in Figure 4C and the Figure 4 legend in the revised manuscript.