Dll4 transduced in tumor cells activates Notch signaling and inhibits endothelial cell proliferation. Delta-like 4 (Dll4) transduction in BL41 cells induced increased expression of Dll4 mRNA (A). Dll4 and vector-only transduced BL41 express EGFP detected by flow cytometry (B). Dll4 transduction induced increased expression of Dll4 protein detected by immunoblotting in BL41 lymphoma cells (C), without affecting BL41 proliferation (D). Dll4-BL41 cells induced Notch activation (E), HEY1/HEY2 expression (F), and inhibited VEGF (25 ng/mL)-induced proliferation (G) in cocultured HUVECs. Bars represent the mean plus or minus SEM (n = 3).