Vascular network formation in CBP homozygous mutant embryos. (A and B) E9.5 whole embryos stained with anti–PECAM-1 antibody to visualize all vessel endothelial cells. Note lack of vascular network formation in CBP homozygous mutant embryos. (C and D) Higher magnification of the head region of E9.5 embryo. Note lack of large vessels and smaller vascular branches in the homozygous embryo. (E and F) Higher magnification of the P-Sp region of E9.5 embryos. Note poor vascular network formation of P-Sp region in CBP homozygous mutant embryos.