Six kilobases of the hEPCR 5′-flanking region transcriptionally targets ECs in transgenic mice. Transgenic mice containing 6 kb of the hEPCR 5′-flanking region coupled to LacZ were generated. Embryos (A-I) and adult tissues (J-L) were analyzed. Embryos were harvested at E12.5 and whole-mount stained with X-gal (A-B). Thereafter stained embryos were wax embedded, sectioned, and counterstained with eosin (C-I). Sites of LacZ expression are dark blue (green arrows). In whole-mount stained embryos, staining in the umbilical vessels was apparent (A-B), which was confirmed as endothelial in tissue sections (C). Transgene expression was also specifically detected in ECs of large placental veins (D), aorta (E-F), and certain other larger veins and arteries (G). EC-specific expression was also seen in small vessels surrounding the spinal cord (H) and in the developing brain (I). Tissues from adult transgenic mice were stained with X-gal and counterstained with hematoxylin (J-L). Reporter gene expression was specifically detected in the endothelium lining the aorta and vena cava (J-L). Microscope images were captured using a Nikon 55i microscope coupled to a DSL1 camera and EclipseNET software (Nikon, Knighton-upon-Thames, United Kingdom). Objectives used were 1×/0.75 NA (panels A, B), 20×/0.75 NA (panels C, D, F-I, K, L), and 2×/0.75 (panels E, J).