Since vessel density is known to be increased in the mammary glands of tumor-bearing mice [22], Notch4 mRNA levels were expressed relative to three endothelium-specific genes: Tie2, VE-cad, and CD31. We found that the levels of Notch4 mRNA were increased in the mouse mammary tumor tissue relative to normal glands (Figure 1c). Here, NOTCH4 is linked to breast cancer.