Fig. 9. Whole-mount (A-D) and section (E-H) analysis reveals that mutation of the Hox site results in activation of the enhancer outside the pro-valve endocardial cells. Activity of the mutated enhancer is observed in umbilical cord (uc), intersomitic artery (isa) (A) and the head vasculature (C). Sectional analysis shows lacZ expression in the endothelial cells of head vasculature (inset in C), ductus venous of E12.5 (F, arrow) and E11.5 embryos (H, arrowhead in inset). Within the heart, aberrant enhancer activity was observed in the endocardial cells of the trabeculated ventricular outlet in E12.5 (E, arrowhead in the inset) and E11.5 hearts (G, marked by an asterisk), and sinus venous valves (G, arrowhead). The dysregulated enhancer activity was also observed in the cushion mesenchymal cells (E, inset). Activity of the mutated enhancer appeared to be increased in the pro-valve endocardial cells (B and D, arrowheads). (I) A table summarizes transient transgenic experiments with the mutated constructs. TG, transgenic embryos; EC, endocardial cell expression; ET, ectopic expression.