Tamoxifen-inducible endothelial-specific activation of the R26R allele by the endothelial-SCL-Cre-ER^T transgene during embryonic development. (A) DNA construct used to generate transgenic mice. The 6.1-kb murine SCL genomic fragment (from -7 to -0.9 kb relative to the transcriptional start in exon Ia) was cloned upstream of the SV40 minimal promoter (SV) and the Cre-ER^T cassette including a rabbit β-globin intron. (B) LacZ-stained end-SCL-Cre-ER^T;R26R embryo (E9.5) after maternal tamoxifen injections at E7.5 (1 mg) and E8.5 (1 mg). (C) E11.5 end-SCL-Cre-ER^T;R26R embryo (maternal tamoxifen injections: E8.5, 0.5 mg; E9.5, 1 mg; E10.5, 2 mg). (D) Magnification of the head region demonstrating LacZ-positive blood vessels on the surface of the telencephalon. (E) Lateral view showing LacZ staining in small capillaries of the forelimb, footplate, and tail. (F) Visceral side of the yolk sac of an E13.5 embryo showing LacZ-positive branching vessels with adjacent smaller capillaries. (G) Histologic section showing LacZ expression in capillaries of the embryonic dermis (E17.5). Isolated LacZ-positive cells may represent cells of hematopoietic origin. (F-G) Maternal tamoxifen injections at E9.5 (0.5 mg), E10.5 (1 mg), and E11.5 (2 mg). (B-F) LacZ whole-mount staining. Original magnification × 40 for panel G.