ECE1 enhancer activity is restricted to arterial endothelium at E12.5. Immunostaining of transverse sections through the trunk region of ECE[623]-lacZ transgenic embryos collected at E12.5. Endogenous Ece-1 protein expression (A) is present in arteries and veins; β-galactosidase protein expression from the ECE1[623]-lacZ transgene (Ac) is restricted to arteries. β-galactosidase protein expression (Bc,Cc) overlaps with expression of the arterial markers Nrp1 (B) and Cx40 (C). β-galactosidase expression (D’,E’) is found only in arteries and is mutually exclusive from the expression of the venous markers Nrp2 (D) and COUP-TFII (E). Respective merged images for each of the markers are shown in A″-E″. A, artery, V, vein. Scale bars, 100 μm.