Reduced Tie-2 expression in Flk1 p/e–HIFdn transgenic embryos. (A) RNA was isolated from wild-type (WT) and transgenic (TG) embryos at E10.5. RT-PCR (+) and a control reaction without RT (–) were performed. No difference in the transcription levels of VE-cadherin, PECAM-1, Tie-1, or VEGF was observed. Flk-1 mRNA was reduced in transgenic embryos, and Tie-2 was below detection level after 35 PCR cycles (n = 4). (B) Immunohistochemistry of embryo sections. Tie-2 was detectable in endothelial cells of wild-type embryos (arrows, top panel) but was absent from vessels in transgenic littermates (arrows, bottom panel). v indicates brain ventricle; L, lumen. Bar in panel B, 50 μm.