shRNA gene therapy inhibits tumor cell proliferation in situ. Illustrated are representative tumor sections prepared 3 weeks after treatment from mice receiving either control vector (A), Del1-shRNA (B), VEGF-shRNA (C), or a combination of Del1-shRNA plus VEGF-shRNA plasmids (D). Tumor sections were stained with anti-Ki-67 Ab to detect proliferative cells. (E) The Ki-67 positive cells were counted to calculate the proliferation index. A significant difference in proliferation index between tumors treated with Del1-shRNA or VEGF-shRNA compared with control is denoted by "*", and a highly significant difference between Del1-shRNA + VEGF-shRNA versus control by "**", and a significant difference between the combinational therapy and Del1-shRNA or VEGF-shRNA monotherapies by "‡".