Dll4 overexpression in tumor cells reduces tumor angiogenesis. Tumor vasculature identified by immunohistochemical staining of mouse CD31 in representative BL41 (A), MOPC315 (C) tumor sections, and quantification of CD31 staining in individual tumors and group means using NIH ImageJ software (B,D). Striped bars represent the mean plus or minus SEM (B: n = 10; D: n = 9).