treatment of human disease. Initial studies focused on the use 
of retroviral vector delivery. An amphotropic retroviral vector 
was used to infect porcine vascular cells in vivo (1) . 
Supernatant from a /9-galactosidase transducing retroviral 
producer cell line (2) , proven to be free of replication 
competent virus (10 5 -10 6 virus particles/ml) , was introduced into 
a localized segment of the iliofemoral artery in pigs using a 
double balloon catheter, and expression of the marker gene was 
confirmed up to five months following transduction. 
Immunohistochemical staining demonstrated that j8-galactosidase 
was expressed within endothelial cells and vascular smooth muscle 
cells. No evidence of replication-competent virus was detected, 
and gene expression was localized to the target site (1) . 
Liposome/DNA complexes have provided an alternative method 
for direct gene transfer into tissues in vivo (3,4). One 
advantage of this method is that any plasmid containing a gene in 
a eukaryotic expression vector with appropriate regulatory 
sequences can be easily utilized. Liposomes can obviate the need 
to synthesize a retroviral vector plasmid, establish subclones of 
retroviral producer cell lines, test them for viral titer, and 
assess the presence of replication-competent helper virus. We 
have employed liposomes complexed to plasmid DNA, containing /3- 
galactosidase or other genes, to transduce localized arterial 
segments. This method of delivery also provides high levels of 
recombinant gene expression localized to specific sites in vivo . 
These experiments employing direct gene transfer in vivo are 
described (1). 
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