Fig. 7. Heterozygous Nkx2-5Y-A ES cell chimaeras show cardiac defects. (A) Control chimaeric heart at 14.5 dpc produced from an Nkx2-5^lacZ/+wild-type ES cells. (B-D) Three examples of Nkx2-5^GFP/Y-Awild-type chimaeras at 14.5 dpc, the section in D stained for lacZ. Arrows in B and C show finger-like projections from the myocardium. (E,F,I,J) Section in situ hybridisation for Myl2 transcript showing low Myl2 transcript levels across the chimaeric heart (F) and in finger-like projections (J), and epicardial blistering (I). (G,H) Enlargements of boxed area in B showing finger-like projections that contain mutant Y-A cells (lacZ staining in H). (K-P) Histological examination of control Nkx2-5^lacZ/+wild-type chimaera (K,O) and mutant Nkx2-5^{Y-A:IRESlacZ/+}wild-type chimaeric hearts (L-N,P) highlighting right ventricular non-compaction (black arrows, L), fenestration of the ventricular septum (red arrow, L), ventricular septal defect (broken arrow in M), tricuspid valve showing Ebstein's anomaly (arrowhead, M; brackets, P) and transmural lesion (arrow, N). Abbreviations: IVS, interventricular septum; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.