Fig. 5. Deletional analysis of the endocardial enhancer. (A) Schematic depicting the unique features of the 4.1 kb BssHII-BssHII intron 1 region and the deletional reporter constructs (d1-d7). (B) Representative whole-mount staining of E11.5 embryos demonstrates the presence of the pro-valve endocardial enhancer activity in d1-d5 but not d6 and d7. (C) Cross-sectional analysis of d5 transgenic embryos (E11.5) shows that the enhancer activity is exclusively restricted to the pro-valve endocardial cells in AVC (arrow) and OFT (arrowhead). The enhancer is not activated in the transformed cells of the endocardial cushions and endothelial cells outside of the heart.