Transgenic expression of Shox2a, but not Shox2a‐DM, mutant in mouse heart results in down‐regulation of Nkx2.5 and cardiac malformation. A P0 control mouse heart shows its gross morphology (A), absent EGFP expression (B), and immunostaining of Nkx2.5 (C). A P0 cTnt‐Cre;pMes‐Shox2a (Shox2aCO) mouse heart exhibits gross blunt heart apex (D), EGFP expression (E), and reduced level of Nkx2.5 protein (F). A P0 cTnt‐Cre;pMes‐Shox2a‐DM (Shox2a‐DMCO) shows similar gross morphology (G) and unaltered Nkx2.5 expression (I), as compared to controls. Transgene expression is revealed by EGFP expression under UV light (H). J, Real‐time RT‐PCR assays show transcriptional reduction of Nkx2.5 expression in the heart from Shox2aCO transgenic mice at P0 (n=3). In contrast, the level of Nkx2.5 mRNA expression in the heart from Shox2a‐DMCO transgenic mice (n=4) remains the same as control (n=5). *P<0.05. Scale bars in red represent 1 mm and scale bars in black represent 100 μm. EGFP indicates enhanced green fluorescent protein; PCR, polymerase chain reaction; WT, wild type.