Fig. 1. (A,B) At stage 30, chick Tbx5 (A) is expressed in the developing left ventricle and Tbx20 (B) is expressed in the right ventricle. The ventricular septum is formed at the boundary of the two areas (arrowheads, A,B) that give rise to the left and right ventricles. (C,D) Expression of chick Nkx2.5 (C) and GATA4 (D) is observed throughout the developing heart ventricles. (E) A small indentation can be observed on the surface of the normal heart (arrowhead) at E5. This corresponds to the position of ventricular septum formation. (F) When Tbx5 was misexpressed, this indentation was not formed. (G) In electroporated hearts, Tbx5 was misexpressed ubiquitously, as confirmed by the GFP signals derived from the electroporated Tbx5-EGFP fusion gene. (H) At E8, the electroporated heart exhibited a round, smooth shape, whereas an indentation (arrowhead) was formed on the surface of the normal heart. (I) In electroporated hearts, GFP fluorescence signals were observed in the entire ventricle. No GFP signal was detected in the normal heart. (J) Normally, the chick atrial natriuretic factor (ANF) gene is expressed in the left ventricle. An indentation can be observed at the right border of ANF gene expression (arrowhead). When Tbx5 was misexpressed, the ANF gene was induced in the entire ventricle at E6 and (K) expression of Tbx20 gene disappeared from the right side where normally this gene is expressed.