© 2009 Springer Science+Business Media B.V. Gene therapy can be classified according to the vector used for gene therapy and the transgene that will be expressed as a result of the gene therapy. One consideration for gene therapy is that certain vectors have larger capacities than others to incorporate genes. The second consideration is the duration of therapy, which depends upon the immune response after delivery of the therapy. Therapy duration usually exhibits a reciprocal relationship to therapy immunogenicity. A third consideration is safety. This is related both to the immunogenicity and adverse effects of potential integration. These factors are shown in Table 1. Advances to enable gene integration with safety have been carried out using a suicide gene, such as thymidine kinase (TK) that can be upregulated to eliminate cells in which the transgene has integrated into an adverse position in the genome.