Both DLL4 and NOTCH4 are strongly expressed in the vascular endothelium and DLL4 is able to activate NOTCH4 signaling (Shutter et al. 2000, Shawber et al. 2003, Shawber et al. 2007). In mice, Notch4 and Dll4 are specifically expressed in arterial endothelial cells, and Dll4 is required for normal arterial patterning and development (Duarte et al. 2004).