We used tcf21:DsRed2 transgenic reporter zebrafish to confirm whether shha mRNA expression overlap with tcf21+ epicardial cells. In the developing heart, we detected shha mRNA expression (arrowheads) in the ventricle (ve), and confirmed that some expression in the ventricle was colocalized with tcf21-driven DsRed2 (arrows, Figure 4—figure supplement 1C). We also detected shha mRNA expression in the bulbus arteriosus (ba) but did not observe colocalization of tcf21-driven DsRed2 with these shha mRNA signals (arrows, Figure 4—figure supplement 1B). These signals may arise from non-epicardial cells. The expression of shha mRNA was also detected in non-cardiac tissues such as the skin and the epithelium of the esophagus the epithelium of the esophagus (es; Figure 4—figure supplement 1A).