Fig. 7. In the wild-type heart, Tbx5 is expressed in the prospective left ventricle (red), not in the prospective right ventricle (yellow). At the boundary of these Tbx5-positive and -negative regions, an interventricular septum was induced. (Type 1) When Tbx5 is misexpressed in the entire ventricle, this boundary is not formed and septum formation does not occur, resulting in a single ventricle. (Type 2) When Tbx5 expression expands to the right, leaving a small Tbx5-negative region, the ventricular septum is shifted to the right. In such hearts, the left ventricle expands, and the right ventricle shrinks. (Type 3) When misexpression of Tbx5 is more restricted to the right ventricle, an ectopic boundary of Tbx5-positive and -negative regions is formed, inducing the ectopic formation of the ventricular septum in the right ventricle. (Right panels) In zebrafish heart, tbx20 is expressed in the bulbus arteriosus (BA) and the ventricular wall near the BA. This gene shows a gradient of expression in the ventricle with strongest signals at the BA, and ending near the atrioventricular junction (AVJ). By contrast, expression of tbx5 is seen in the AVJ and the ventricle near the AVJ, but expression becomes faint near the BA. This indicates that tbx20 and tbx5 are expressed in a complementary fashion, but not discreetly. Expression patterns are basically same at 48 and 120 hours post fertilization (hpf).