Dll4 can be transferred to mouse endothelial cells in vivo. (A) Immunohistochemical staining of hDll4 in control tumors derived from U87 cells or tumors derived from U87 overexpressing Dll4. hDll4 staining was not detectable in mouse tissue (red arrow); however, hDll4 staining was detectable in tumor tissue (white arrow) and in mouse vessels (yellow arrow). (B) Immunohistochemical staining of serial sections derived from a U87 Dll4 tumor to detect mouse CD31 and human Dll4. The cells surrounding the vessel were positive for CD31 and hDll4, which confirms that the mouse endothelial cells had acquired the hDll4 from the tumor cells. Images were taken with a Nikon Eclipse E800 microscope at 40× magnification.