It is evident that the vectors in current use are not ideal for in vivo gene delivery, especially for the targeted transfer of exogenous genes to multifocal metastatic lesions. In view of the insufficiently high titres of current viral vectors it is therefore almost obligatory to employ therapeutic genes whose products would produce an effect even when only a small proportion of the target cells express the gene. Currently, most of the work involving viral vectors has employed replication-defective viruses. However, there are persuasive reasons to consider the use of replication-competent viruses for in vivo gene transfer, where the therapeutic gene would then be transmissable to other cells, resulting in amplification of the number of cells expressing the transgene . Such an approach would absolutely require reliable strategies to target the vectors to the tumour cells or to target transgene expression.
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