LacZ expression in the yolk sac at 13.5 dpc. (A) lacZ expression in Flk1⁺/lacZ neo-in embryo. Note small blood vessels between arteries and veins are not stained well after 24 hours of X-gal staining. (B) Expression of lacZ Flk1⁺lacZ neo-out embryo after 4 hours of X-gal staining. (C) Expression of lacZ in Flk1⁺/lacZ del Sw/B neo-out embryo. Note that most blood vessels are stained well after 4 hours of X-gal staining. (D) Immunostaining of a wild-type yolk sac using anti-Flk1 antibody. Note that Flk1 antibody stains most blood vessels.