Cell line issues related to specific expression vectors: retrovirus vectors.

Retroviruses have received attention as vectors for gene transfer because they can infect a variety of cell types, they integrate stably into the genome of the target cell and they have simple, well-defined structures. Insertion of the gene of interest into a retroviral vector results in the formation of replication-defective particles that can be produced in "packaging" cell lines. Due to the nature of the present generation of vectors and packaging cell lines, multiple recombination events would be needed to produce replication-competent particles. However, the concern still exists that replication-competent virus could arise either in the production of retroviral vector stocks or following their introduction into target cells. The current use of retrovirus vectors and the challenge they present to testing programmes are discussed.