Impaired expression of endocardial and cushion mesenchymal genes in Msx1-/- ; Msx2-/- mutant AV cushions. At E10.5, the immunostaining intensity of NFATc1 was reduced in Msx1-/-; Msx2-/- mutant AV endocardium compared with the littermate control (red arrows in A, B), while the intensity was comparable between control and Msx1/2 double mutant OFT endocardium (red asterisks in A, B). At the same developmental stage, RNA section in situ hybridization revealed greatly diminished expression of Notch1 (arrows in C, D) in the Msx1/2 double mutant AV cushion mesenchyme and endocardium compared with the control. In contrast, Notch1 expression was normal in the Msx1/2 mutant OFT cushions (compare red asterisks in C and D). Note also a decreased number of Has2-expressing cells in the Msx1/2 null mutant AV cushion mesenchyme (red arrows in E, F). endo, endocardium; LA and LV, left atrium and ventricle; mes, mesenchyme; RA and RV, right atrium and ventricle. Scale bars: 0.1 mm in B (for A-D), 0.2 mm in F (for E, F).