['To evaluate whether sustained expression of mouse endostatin by adeno-associated virus (AAV)-mediated gene transfer can enhance the treatment efficacy of ionizing radiation.', 'Mouse endostatin was cloned into recombinant AAV (rAAV) under the control of CMV beta-actin promoter. Recombinant mouse endostatin expressed via AAV gene transfer was tested for biological activity in endothelial cells. The impact of elevated serum levels of endostatin on tumor-induced angiogenesis was evaluated using an in vivo angiogenesis assay. The anti-tumor efficacy of combining rAAV-mediated endostatin delivery with radiation was evaluated in a human colorectal tumor model (HT29).', 'Recombinant mouse endostatin expressed through an AAV vector (rAAV-mEndo) inhibited endothelial cell proliferation (by 40-45%) and migration (by 22-33%). Intramuscular injection of rAAV-mEndo (1x10(9) i.u.) led to a sustained serum endostatin level of approximately 500 ng/ml. Compared to control animals this endostatin level was sufficient to inhibit tumor cell-induced vessel formation (37 vs. 28.5, P<0.05) and delay the growth of HT29 xenografts (time from 200 to 1,000 mm(3), 21 vs. 34.5 days, P<0.05). When combined with ionizing radiation, elevated serum endostatin levels significantly enhanced the time for tumors to grow from 200 to 1,000 mm(3) (radiation, 34 days; endostatin plus radiation, 50 days, P<0.05).'] Does gene therapy delivery of endostatin enhance the treatment efficacy of radiation?