Partial structure and functional analysis of the mouseFlk-1 locus. (A) Restriction enzyme map of the region encompassing the first three exons (represented by shaded boxes). Subfragments containing parts of intron 1 or intron 2 are indicated. Abbreviations for restriction enzymes are: B, BamHI, Xh,XhoI, Sl, SalI. (B) β-galactosidase reporter gene assays of various constructs after transient transfection of BAE cells. The intron fragments were tested in combination with a 4.4 kbpFlk-1 promoter fragment spanning the region from −4.1 kbp to +299 bp of the Flk-1 gene. NIH 3T3 cells were used as a reference for nonendothelial cells. Values significantly above (P < .003, Student’s t-test) 100 RLU are marked with an asterisk.