Although Ad5H2E-PPE1(3x)-GFP or Ad5H2E-mVEGFR2-GFP conferred GFP reporter expression in only 5–6% of total tumor endothelium, the large anti-tumor effect following infection of such a small fraction of the tumor microvascular endothelium can be explained by the secretory nature of the asmase gene product, analogous to the bystander effect seen in many types of gene therapy [31]–[36]. Here, SMPD1 is linked to infection.