Panels A and B show Crtl1 mRNA expression by whole mount in situ hybridization in the developing mouse heart at ED10.0 (panel A) and ED12.0 (panel B). At ED10.0, Crtl1 mRNA is abundantly expressed in the AVJ and conal region of the OFT (*). Relatively high expression is also seen in the atrial roof. Lower levels of expression are seen in the lining off the atria and ventricles. A ED12.0 (panel B) Crtl1 mRNA expression becomes more restricted, with high expression in the AVJ and OFT (*). At this stage, Crtl1 mRNA expression is no longer detected in the atria and ventricles. LA=left atrium, RA=right atrium, LV=left ventricle, RV=right ventricle, OFT=outflow tract, AVJ=atrioventricular junction, (*)=OFT conal cushions.