Perturbed expression of chamber-specific genes in Msx1-/- ; Msx2-/- mutant myocardium. RNA section in situ hybridization at E10.5 demonstrated that Tbx2 expression was dramatically reduced in the Msx1/2 null mutant AV myocardium compared with the littermate control (compare the staining intensity in the regions pointed by red notched arrowheads in A and B). In contrast, the level of Tbx expression in the pharyngeal mesoderm was comparable between the control and the Msx1/2 double mutant (indicated by black arrows in A and B). Concomitant with decreased Tbx2 expression in the Msx1-/-; Msx2-/- mutant AV myocardium, Anf expression exhibited increased expression in the Msx1/2 double mutant AV myocardium (compare the staining intensity in the regions pointed by red notched arrowheads in C and D). In addition, we detected ectopic Anf expression in the double mutant right ventricle (red asterisk in D), which was likely a hemodynamic effect secondary to the changes of gene expression in the AV canal. Msx1/2 null mutants also exhibited decreased expression of Hand1 (F) and Hand2 (H) in the AV myocardium (red notched arrowheads). In the Msx1/2 mutant AV myocardium, Hand1 expression was almost undetectable and Hand2 expression was significantly reduced compared with the control (red notched arrowheads in E-H). LA and LV, left atrium and ventricle; RA and RV, right atrium and ventricle. Scale bars: 0.1 mm in A (for A-D) and E (for E-H).