To examine the effect of endothelial Rac1-depletion on tumor angiogenesis, blood vessel density was quantified by counting the numbers of PECAM-1+/GFP− blood vessels per unit area across entire midline tumor sections from size-matched tumors of wild-type/Tie1-Cre+ and β3-null/Tie1-Cre+ mice treated with either pSico-Con, pSico-Rac1 or pSico-Flk-1 lentiviruses. The gene discussed is PECAM1; the disease is neoplasm.