The "porto-cava" gradient of the transgene expression viewed on a 50 × magnification field of liver section stained for X-Gal and counterstained with red. The livers were collected 2 days after iv injection of 1010 iu Ad5 Adapt LacZ A: Brown Norway rat. B: Wag/Rij rat. The white arrow points to a interlobular (portal) vein. The black arrow points to a central vein in the centro-lobular area.