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). The gene discussed is PECAM1; the disease is breast cancer.