Fig. 4. Analysis of ß-galactosidase activity in embryos from four independent stable transgenic lines shows consistent pro-valve endocardial enhancer activity of the 4.1 kb BssHII-BssHII P2 fragment in whole-mount-stained E8.5 to E12.5 embryos (A-E). X-gal staining is restricted to the endocardial cells in AVC (arrow) and OFT (arrowhead), especially intensified at later stages in the regions of forming valves and septa. (F-J) Sectioning of stained E8.5, E9.5, E12.5 and E14.5 embryos highlights this enhancer activity for the pro-valve endocardial cells of the forming valves. ao, aorta; av, aortic valve; pt, pulmonary trunk; pv, pulmonary valve; la, left atrium; ra, right atrium; lv, left ventricle; rv, right ventricle; mv, mitral valve; tv, tricuspid valve.