Generation of Flk1 p/e–HIFdn transgenic embryos. (A) Expression vector for the generation of HIFdn transgenic mouse embryos. (B) Well-vascularized yolk sac of a wild-type embryo at E12.5 (arrow indicates branching vessels). (C) The yolk sac of a transgenic littermate was not perfused and lacked large vessels. (D) Wild-type embryo at E10.5 with well-organized cephalic vascular tree (arrow). (E) HIFdn transgenic embryos at E10.5 were growth retarded, had a disorganized vasculature (open arrow), and showed hemorrhages (filled arrows). (F) In transgenic embryos, HIFdn was detected by RT-PCR in the RT reaction (+) but not in control reaction without reverse transcriptase (–). (G-H) FLAG-tagged HIFdn was detected by immunohistochemistry in blood vessels of transgenics (H, arrows) but not in vessels of wild-type embryos (G, arrow). L indicates vessel lumen; bar, 50 μm. (I) Transgenic embryos at E11.5 displayed pericardial edema (arrow). Magnification: 8 × (B,C), 12 × (D,E,I).