Msx1 and Msx2 are essential for the development of many organs. In the heart, they act redundantly in development of the cardiac cushions. Additionally, Msx2 is expressed in the developing conduction system. However, the exact expression of Msx1 has not been established. We show that Msx1 is expressed in the cardiac cushions, but not in the myocardium. In Msx2-null mice, Msx1 is not ectopically expressed in the myocardium. The absence of myocardial defects in the Msx2 knock-out can therefore not be attributed to a redundant action of Msx1 in the myocardium. Anat Rec, 2010. © 2010 Wiley-Liss, Inc.