Analysis of transgene expression during early development and in newborn mice in the transgenic mouse line 2603. (A) Frontal view on a whole-mount β-galactosidase–stained E7.8 embryo. The arrow indicates transgene expression in the extraembryonic mesoderm. (B and C) Paraffin sections from the embryo shown in A show transgene expression in endothelial cells of the allantois (B) and the yolk sac (C). (D to H) LacZ staining of spleen (D), kidney (E), lung (F), liver (G), and thymus (H) from a P5 transgenic mouse. (EM) extraembryonic mesoderm. Bars, 25 μm (C), 100 μm (B,D,E,F,G,H).