Effect of exogenous Rae28 on Nkx2.5 expression in the heart. Expression of the exogenous Rae28 genes and their effect on Nkx2.5 expression were examined by whole-mount in situ hybridization with Rae28 (a–c) and Nkx2.5 probes (d–g). Ubiquitous and cardiomyocyte-specific Rae28 expression was detected at 8.5 dpc in Rae28^–/–/RSV-Rae28 and Rae28^–/–/βMHC-Rae28 embryos, respectively (b and c), either of which possessed a genetic background of Rae28^–/– where Rae28 expression was undetectable (a). Ubiquitous Rae28 expression efficiently reverted reduced expression of Nkx2.5 in the Rae28^–/– embryo (e), while cardiomyocyte-specific expression of Rae28 neither augmented nor reduced Nkx2.5 expression in the wild-type heart (f) and did not exert any effect on reduced expression of Nkx2.5 in the Rae28^–/– embryo (g). We further confirmed these findings in other embryos (data not shown). WT, wild-type embryo; βMHC, βMHC-Rae28; –/–&RSV, Rae28^–/–/RSV-Rae28; –/–&βMHC, Rae28^–/–/βMHC-Rae28. Scale bar: 200 μm (a–c), 500 μm (d–g).