Dll4-expressing endothelial cells activate Notch1 on adjacent ovarian tumor cells. Immunohistochemical staining for active Notch1 (nuclear Notch1 intracellular domain) and the tumor cell marker vimentin demonstrates Notch1 signaling activity in the tumor vasculature (elongated nuclei) and in vimentin-positive, tumor vessel-associated parenchymal cells. This pattern illustrates the important concept of juxtacrine signaling interactions between Dll4 expressed by endothelial cells and adjacent, Notch1-positive ovarian tumor cells. For details, see article by Kuhnert and colleagues on page 4086.