Iintroduction

Gene therapy aims to treat both genetic and infectious diseases by the introduction of new genetic material into the appropriate cells in the body (1). In the simplest case of a defective gene causing disease, addition of the new gene will restore function. Alternatively the new genetic material can be designed to selectively kill a tumor cell, to induce an immune response, or to protectively ''immunize'' a cell against an incoming pathogen. Nontherapeutic uses of gene therapy include gene marking, which has proved especially useful in identifying the sources of recurring malignancies in autolo-gous bone marrow transplant patients. These potential applications of gene therapy are described in Table 1.

The first approved gene therapy clinical protocol began in September 1990, using retroviral vectors to introduce copies of the adenosine deaminase (ADA) gene into T cells from a patient with ADA deficiency (2,3). In 2002, over 600 clinical protocols have been approved worldwide, incorporating over 3500 patients. Most of these trials have used viral vectors, with the largest number (34% of trials, 50% of total patients) involving retroviral vectors. This chapter explores the features of retroviral vectors that make them so versatile for gene therapy and highlights both their current limitations and potential improvements.

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