A 430-bp fragment from the

Flk-1 first intron is sufficient for endothelium-specific reporter gene expression in transgenic mice.(A) Partial structure and restriction-enzyme map of the murineFlk-1 locus. Exons are represented by shaded boxes. The positions of the 939-bp promoter fragment and the 510-bpSwaI/BamHI enhancer fragment are indicated. Abbreviations for restriction enzymes are Xh, XhoI; Sw,SwaI; B, BamHI. (B) Structure of reporter gene constructs. The LacZ reporter gene (blue) is flanked by aFlk-1 promoter fragment spanning bp −640 to bp +299 (red) and subfragments of the 510-bp SwaI/BamHI intron fragment (green). Transcription terminates at a simian virus (SV40) polyadenylation signal (yellow). (C) Structure of intron fragments and their position in the 510-bp SwaI/BamHI intron fragment. Fragment lengths are indicated. (D) Whole-mount LacZ-stained embryo. This embryo is transgenic for a reporter gene construct in which the LacZ gene is under control of the 939-bp Flk-1promoter and the 430-bp enhancer fragment.