BMPER protein expression was evaluated in the atrioventricular cushions (AVC; A-C) and outflow tract cushions (OFT; D-F) at E9.5 (A, D), E10.5 (B, E), and E11.5 (C, F). (A-C) BMPER protein is strongly expressed in the mesenchymal cells that populate the AVCs and is expressed at very low levels in the overlying endocardial layer. (D-F) Consistent with epithelial-mesenchymal transition (EMT) occurring slightly later in the OFT cushions compared with the AVCs (A-C), no BMPER-positive cells are observed in the OFT at E9.5 (D). As EMT progresses (E-F), the OFT cushions fill with BMPER-positive mesenchymal cells. Scale bar in D = 60 μm in panels A, B, D, and E and = 120 μm in panels C and F. A, common atrium; V, common ventricle; LA, left atrium; LV, left ventricle.