Advances in molecular virology and biotechnology have led to the engineering of vectors that can efficiently transfer genes to target cells. Gene therapy strategies were developed along two lines: Cytotoxic approaches involve the transfer of genes that encode enzymes, which convert inactive prodrugs into cytotoxic drugs. Corrective gene therapy approaches aim to repair specific molecular alterations in signal transduction mechanisms that control the cell cycle or induce apoptosis. Clinical evidence suggests that gene therapies are best suited for patients with minimal residual disease. Multimodality approaches with conventional strategies and novel therapeutic tools in various combinations will most likely prove advantageous, compared to single-modality treatments. However, clinical trials will need to test these hypotheses.