ABSTRACT
Plasmid-based gene therapy approaches present certain advantages over viral-based approaches. Although viral systems have the capability of delivering transgenes efficiently and expressing them at high levels, they have significant limitations (Verma and Somia, 1997). Limits to the sizes of viral genomes impose constraints on lengths of inserted DNA fragments. Recombinant virus particles must be replicationdefective and produced free from the replication-competent helper viruses that are required for viral packaging and propagation. Scaling up the manufacturing of viral preparations that are high titer and free from helper virus is often difficult, and viral particles must be specially stored to retain infectivity. Strong immune responses against the cells infected by the recombinant viruses contribute to transient expression and inhibit the implementation of therapies that require recurrent administration.
