^–5.5HS functions as an autonomous endothelial enhancer in transgenic mice. Transgenic embryos generated with LacZ under the control of –5.5HS/SV40P were harvested at approximately E12.5, whole-mount stained with X-gal, sectioned, and counterstained with eosin. Sites of LacZ expression are dark blue (green arrows). In 4 of 4 transgenic embryos reporter gene expression was specifically detected in ECs, including those of the aorta and vena cava (A-B). In 1 embryo, very strong staining of the endocardium lining the chambers of the developing heart was seen (A,C). Transgene expression was also specifically detected in ECs of umbilical vessels (D), small vessels surrounding the spinal cord (E), and in the hepatic vein (H). In 2 of 4 embryos, strong staining in a small number of bone marrow (BM) cells was seen (E-G). Microscope images were captured using a Nikon 55i microscope coupled to a DSL1 camera and EclipseNET software (Nikon). Images were processed using Adobe Photoshop (Adobe Systems, San Jose, CA). Objectives used were as follows: 4×/0.75 NA (A, E) and 20×/0.75 (B-D, F-H).