Arterial maker expression is severely reduced in Dll1 mutant embryos. At E13.5, mutant embryos still show normal expression of arterial markers comparable to wild-type embryos (A, B, G, H, M, N, S, T). At E15.5 and E18.5, the arterial markers Neuropilin1 (C-F), VEGFR2 (I-L), EphrinB2 (O-R) and Smoothelin (U-X) are severely reduced in arterial vessels. The venous marker COUP-TFII, which is normally expressed in perivascular cells surrounding arteries, is ectopically up-regulated in arterial endothelial cells (Y-ZD) at E15.5 (arrowheads in ZB′) and E18.5 (arrowheads in ZD′). Green indicates α-smooth muscle actin; red indicates antigen. A′ to ZD′ show magnifications of the boxed areas indicated in A-ZD.