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Fig. 8 Constitutive MEK1 activity induces ectopic vmhc expression in the atrial myocardium. (A-R) Fluorescent in situ hybridization shows expression of vmhc (green) together with immunofluorescence using MF20 (red, A-I) or labeling Amhc (red, J-R) at 48 hpf, following heat shock at 16 hpf. Three-dimensional reconstructions of frontal views (A-F,J-O), as well as single optical sections (G-I,P-R), highlight the localization of vmhc expression within the atrial myocardium in embryos expressing Tg(hsp:caMEK1) (D,E,G,I,M,N,P-R; arrowheads). In these experiments, all examined Tg(hsp:caMEK1) embryos exhibited ectopic vmhc, in contrast to the incomplete penetrance documented in Fig. 7; this difference is likely due to the heightened sensitivity of fluorescent in situ hybridization. (A-C) n=6, (D-I) n=8/8, (J-L) n=5; (M-R) n=10/10. Scale bars: 50 µm.