(A) Diagram of the extended IrxA cluster showing the location of Irx1, Irx2, Irx4 and the neighboring genes Ndufs6, Mrpl36 and Lpcat1 (not to scale). (B-J) In situ hybridization in sections of control and mutant hearts for Irx4 at E9.5 (B), E10.5 (C) and E11.5 (D); and Irx1 (E), Irx2 (F) and Ndufs6 (G) at E11.5. (H-I) Higher magnifications (black dashed boxes) of Irx1 (H), Irx2 (I) and Irx4 (J) expression in control and mutant hearts at E11.5. Arrows indicate extension in the expression territories of Irx1 (H) and Irx2 (I). A,atria; V, ventricle, RA; right atria; LA, left atria; RV, right ventricle; LV, left ventricle; AVC, atrioventricular canal; IVS, interventricular septum. Scale bars, 100 μm (B), 200 μm (C-J).