Effects of Dll4 overexpression in tumors resistant to VEGF blockade. Growth curves (A) and weight (B) of Dll4 and control WEHI3 tumors; tumors were removed 17 days after subcutaneous injection of WEHI3 cells in BALB/c mice. Solid bars represent individual tumor weights and striped bars represent group means plus or minus SEM (n = 10). Vasculature identified by immunohistochemical staining of mouse CD31 in representative WEHI3 tumors (C), and quantification of CD31 staining in individual tumor sections and group means (D). Solid bars reflect individual tumor sections and striped bars represent the mean plus or minus SEM (n = 6). Human Dll4 (E), mouse HEY1 (F), and mouse HEY2 (G) mRNA levels measured by real-time PCR in WEHI3 tumor extracts. Bars represent the relative mean plus or minus SEM mRNA levels (n = 10, E-G).